fix(fleet): scope Stop-by-label to stack labels with a typed suggestion source (#1368)

* fix(fleet): scope Stop-by-label to stack labels with a typed suggestion source

The Fleet Actions "Stop by label" card labelled its target field generically
as "Label", so a same-named node label could look like a valid stop target in
a destructive workflow. The action has always matched stack labels only, but
nothing in the copy or the data flow made that explicit.

Add a stack-label-only suggestions endpoint and make the scope unmistakable:

- New GET /api/fleet/labels/suggestions aggregates the per-node stack labels
  into a name-keyed list with stack and node counts (admin-only, central DB,
  covers every configured node including offline remotes). Node labels are
  never folded in.
- The card now sources its autocomplete from that endpoint and renders each
  suggestion with its stack and node counts via a typed FleetStopLabelSuggestion
  model, so node-label data cannot be fed into this destructive card.
- Copy is explicit throughout: "Stack label" target field with a helper line
  that node labels are not used, a clear "0 matching stacks" readout and a
  "No stacks are assigned to this stack label" empty preview, and confirm and
  result copy that references stacks and the stack label.
- Docs updated (fleet-actions, stack-labels) and tests added on both sides,
  including node-only exclusion, name collision, multi-node counts, the
  zero-stack preview, and the non-fatal suggestions-load path.

* docs: correct stale Stop-by-label button and helper references

The Stop-by-label walkthrough referenced a "Stop matching stacks" button and a
warning callout that no longer exist on the card. Align the docs with the live
card: the primary action is "Stop fleet", and the scope is stated by the helper
line under the input.

docs/features/fleet-actions.mdx

@@ -41,30 +41,30 @@ The two fan-out cards (Stop and Prune) iterate every node in **Settings → Node
 
 ## Stop fleet by label
 
-Stop every stack that carries a given label name on every node where that label exists. Labels are matched **by name** across the fleet, so a label called `production` on one node and an independently-authored `production` label on another node both match. See [Stack Labels](/features/stack-labels) for how to author the selector taxonomy.
+Stop every stack assigned a given **stack label** on every node where that stack label exists. Stack labels are matched **by name** across the fleet, so a stack label called `production` on one node and an independently-authored `production` label on another node both match. This action targets stack labels only; node labels (used for node grouping) are never used here. See [Stack Labels](/features/stack-labels) for how to author the selector taxonomy.
 
 ### Step by step
 
 1. Open **Fleet → Actions**.
-2. Type a label name in the **Label name** field. The input autocompletes against label names that already exist on any **online** node; an offline node with an unseen label still receives the request, it just won't show up in the suggestions.
-3. Click **Stop matching stacks**.
-4. A confirmation appears with the kicker **Fleet stop** and the title `Stop all stacks labeled "<name>"?`. Click **Stop fleet** to commit.
+2. Type a name in the **Stack label** field. The picker suggests stack labels from across the fleet, each with its stack and node counts, so the scope is unmistakable. Suggestions are read from the control instance, so stack labels on offline nodes appear too. You can also type a name by hand.
+3. Click **Stop fleet**.
+4. A confirmation appears with the kicker **Fleet stop** and the title `Stop all stacks with the stack label "<name>"?`. Click **Stop fleet** to commit.
 
 <Frame>
-  <img src="/images/fleet-actions/fleet-actions-stop-confirm.png" alt="Fleet stop confirmation dialog. Kicker 'Fleet stop' in red mono, italic title 'Stop all stacks labeled &quot;docs-preview&quot;?', Cancel and Stop fleet buttons in the footer." />
+  <img src="/images/fleet-actions/fleet-actions-stop-confirm.png" alt="Fleet stop confirmation dialog. Kicker 'Fleet stop' in red mono, italic title 'Stop all stacks with the stack label &quot;docs-preview&quot;?', Cancel and Stop fleet buttons in the footer." />
 </Frame>
 
 ### Reading the per-node breakdown
 
-When the request finishes, the results render below the form, grouped by node. Each node row carries a colored icon and either a stack count or a `(no matching label)` annotation; the indented children below each row are the per-stack results.
+When the request finishes, the results render below the form, grouped by node. Each node row carries a colored icon and either a stack count or a `(no matching stack label)` annotation; the indented children below each row are the per-stack results.
 
 <Frame>
-  <img src="/images/fleet-actions/fleet-actions-stop-results.png" alt="Per-node breakdown after running fleet stop against a label that no node has. The header reads PER-NODE BREAKDOWN with two badges, '0 ok' and '7 failed'. Seven rows follow, one per node (Local, Opsix, Pitt-Moba, SLX-Mars, sencho-pilot-test, sencho-test-01, sencho-test-02), each annotated '(no matching label) · Label not present'." />
+  <img src="/images/fleet-actions/fleet-actions-stop-results.png" alt="Per-node breakdown after running fleet stop against a label that no node has. The header reads PER-NODE BREAKDOWN with two badges, '0 ok' and '7 failed'. Seven rows follow, one per node (Local, Opsix, Pitt-Moba, SLX-Mars, sencho-pilot-test, sencho-test-01, sencho-test-02), each annotated '(no matching stack label) · Stack label not present'." />
 </Frame>
 
 A few quirks worth knowing:
 
-- A node that has no label by that name appears as `<node> (no matching label)` and is counted in the **failed** badge. This is not a transport failure, it just means the label was not present on that node.
+- A node that has no stack label by that name appears as `<node> (no matching stack label)` and is counted in the **failed** badge. This is not a transport failure, it just means the stack label was not present on that node.
 - A node where the label exists but no stacks are assigned to it appears with a matched count of zero stacks. No per-stack rows render.
 - When the control instance reaches a remote node, the per-stack result you see comes from the remote node's own response. If the remote returns a non-2xx for the whole label, every stack on that node renders with the same error message.
 
@@ -169,11 +169,11 @@ Run **Prune Docker resources fleet-wide** with **Images** selected and **Managed
 ## Troubleshooting
 
 <AccordionGroup>
-  <Accordion title="A node shows '(no matching label)' but I created the label there">
-    The fleet-stop match is by **label name**, not label ID. Confirm the label name on the affected node under **Settings → Labels**; a typo, a case mismatch, or a trailing space will leave the node out. Labels are scoped per node, so renaming the label on one node does not propagate to the others.
+  <Accordion title="A node shows '(no matching stack label)' but I created the label there">
+    The fleet-stop match is by **stack label name**, not label ID. Confirm the stack label name on the affected node under **Settings → Labels**; a typo, a case mismatch, or a trailing space will leave the node out. Stack labels are scoped per node, so renaming the label on one node does not propagate to the others.
   </Accordion>
-  <Accordion title="The autocomplete didn't suggest a label I know exists">
-    The autocomplete fans out to **online** nodes only and aggregates label names from their label list response. A label that exists only on an offline node won't appear in the suggestions. The fleet-stop request still iterates every configured node, so typing the name by hand and submitting will reach the offline node when it returns.
+  <Accordion title="The autocomplete didn't suggest a stack label I know exists">
+    The picker reads stack labels from the control instance across every configured node, including offline ones, so a name is missing only when no node has a **stack label** by that name. Node labels never appear here, because this action targets stack labels only. You can always type a name by hand; the fleet-stop request still iterates every configured node.
   </Accordion>
   <Accordion title="Bulk label assign reports 'Invalid stack name' for one row">
     Stack names must match the standard validator: alphanumeric plus dash and underscore, no spaces, no path separators. The endpoint validates each assignment independently, so a single bad name does not block the rest of the batch. Fix the offending entry and re-run; the rows that already succeeded won't be re-applied.

docs/features/stack-labels.mdx

@@ -88,14 +88,14 @@ A stack can carry multiple labels and will then appear under each label's group
 Two cards in the **Fleet · Actions** tab use labels to drive cross-node operations. See [Fleet Actions](/features/fleet-actions) for the full reference.
 
 <Frame>
-  <img src="/images/stack-labels/fleet-actions.png" alt="Fleet Actions tab with two cards side by side. Left card 'Stop fleet by label' (rose accent rail) has a Label name combobox containing 'Media' and a Stop matching stacks button beneath. Right card 'Bulk label assign' (purple accent rail) has a node selector reading Local (local), a stacks checklist showing plex and radarr ticked, a Labels row with a highlighted Media pill plus inactive Network and Utilities, and an Apply to 3 stacks button." />
+  <img src="/images/stack-labels/fleet-actions.png" alt="Fleet Actions tab with two cards side by side. Left card 'Stop fleet by label' (rose accent rail) has a Stack label combobox containing 'Media' and a Stop fleet button beneath. Right card 'Bulk label assign' (purple accent rail) has a node selector reading Local (local), a stacks checklist showing plex and radarr ticked, a Labels row with a highlighted Media pill plus inactive Network and Utilities, and an Apply to 3 stacks button." />
 </Frame>
 
 ### Stop fleet by label
 
-Type a label name; Sencho fans the request out to every online node and stops every stack on that node that carries a label with the same name. The card autocompletes the input from the union of label names on every reachable node, so you do not need to remember whose label rows exist where. The result list shows a per-node breakdown with success and failure counts, and the warning callout under the input restates the fleet-wide semantics: `Different nodes can have their own label rows. Stops are dispatched per node and report per-stack results below.` A confirmation modal titled `Stop all stacks labeled "<name>"?` with the **Stop fleet** primary action runs the action.
+Type a stack label name; Sencho fans the request out to every node and stops every stack on that node assigned a stack label with the same name. This action targets stack labels only, never node labels. The picker suggests stack labels from across the fleet, read from the control instance so labels on offline nodes appear too, each with its stack and node counts so the scope is unmistakable. The result list shows a per-node breakdown with success and failure counts, and the helper line under the input states the scope: `Stops stacks assigned to this stack label across matching nodes. Node labels are not used by this action.` A confirmation modal titled `Stop all stacks with the stack label "<name>"?` with the **Stop fleet** primary action runs the action.
 
-Offline nodes are skipped during autocomplete loading and are reported as failures during the actual run, so a partial-fleet stop is observable rather than silent.
+A node with no stack label by that name is reported as a failure during the run, so a partial-fleet stop is observable rather than silent.
 
 ### Bulk label assign
 
@@ -128,8 +128,8 @@ The **Bulk label assign** card is per-node only by design. To re-tag stacks on a
   <Accordion title="The Tags filter does not list a label I just created">
     The Tags filter aggregates labels across every node in the fleet by name. If the new label only exists on one node and that node was offline at the moment the page loaded, the dropdown may not include it. Refresh **Fleet · Overview** with the **Refresh** button in the toolbar to repull node state.
   </Accordion>
-  <Accordion title="`Stop fleet by label` reports `No nodes have a label by that name`">
-    Labels are per-node, so the fleet-stop matches by name across nodes. If the label you typed only exists on the active node and you typed the wrong case (`prod` versus `Prod`), no node will match. The combobox autocompletes from the union of label names on reachable nodes; pick from the suggestion list rather than typing freehand to avoid case mistakes.
+  <Accordion title="`Stop fleet by label` reports `No node carries a stack label by that name`">
+    Stack labels are per-node, so the fleet-stop matches by name across nodes. If the stack label you typed only exists on one node and you typed the wrong case (`prod` versus `Prod`), no node will match. The picker suggests stack labels read from the control instance across every configured node, including offline ones; node labels never appear there. Pick from the suggestion list rather than typing freehand to avoid case mistakes.
   </Accordion>
   <Accordion title="`Bulk label assign` cleared every label on my stacks unexpectedly">
     The card replaces, it does not merge. Selecting no labels and clicking **Apply to N stack(s)** is the documented way to clear assignments, and the confirmation copy on the **Bulk label assign** modal restates this: `No labels selected, this will clear existing assignments on the selected stacks.` Re-pick the labels you want and run the action again to restore them.