refactor(auto-update): retire per-stack gate, drive auto-update from schedules only (#1233)

* refactor(auto-update): retire per-stack gate, drive auto-update from schedules only

The per-stack Auto-update toggle in the stack sidebar context menu wrote a
gate row to `stack_auto_update_settings`, but actual updates only ran when a
`scheduled_tasks` row with `action='update'` fired. On a fresh install the
toggle was inert: detection ran every 6h, nothing was applied.

The same context menu already exposes `Schedule task`, which opens
ScheduledOperationsView pre-filled for the stack where the user can pick
`Auto-update Stack` and any cron. Keeping the toggle alongside that flow
duplicated the same action and turned the gate table into a parallel store
of "is a covering schedule active" derivable from `scheduled_tasks` itself.

Drop the gate model entirely:
- Backend: remove the `stack_auto_update_settings` table and its four
  accessors, the three routes under /api/stacks/*/auto-update, the per-stack
  skip in /api/auto-update/execute and SchedulerService.executeUpdate's
  fleet branch, and the clearStackAutoUpdateSetting call on stack delete.
  Dashboard `autoUpdate` count derives from scheduled_tasks (action='update'
  rows pinned to the node, total/enabled split).
- Frontend: drop the Auto-update entry from the sidebar context menu and its
  optimistic toggle plumbing. Drop autoUpdateSettings state, the
  /stacks/auto-update-settings fetch, and the auto-update-settings-changed
  WebSocket branch. Slim useSidebarActivitySummary (just nextRunAt; no
  enabled/total counts). AutoUpdateReadinessView's per-card autoUpdateEnabled
  now means "a covering enabled action='update' schedule exists" (per-stack
  row or fleet row on this node, earliest next_run_at wins, per-stack row
  wins on ties), with the gate-fetch removed.
- New: scheduledTasksRouter broadcasts scope: 'scheduled-tasks' on POST,
  PUT, PATCH /toggle, and DELETE so useConfigurationStatus and
  useNextAutoUpdateRun refetch under the 250ms debounce instead of waiting
  for the 60s poll. The broadcast is wrapped so a broken subscriber socket
  cannot turn a successful mutation into a 500.
- Docs: rewrite the "Per-stack control" section of auto-update-policies.mdx
  to describe the schedule-based model; update the matching troubleshooting
  entry. The misleading fleet-update help text in ScheduledOperationsView
  is corrected to reflect that every stack on the node is covered.

Tier parity: the surviving auto-update path (Schedule task -> Auto-update
Stack / All Stacks) is gated `requirePaid + requireAdmin` backend and
`isPaid + isAdmin` frontend, matching the gate the deleted routes carried.
The pre-commit grep returns no tier-related diff outside this PR's scope.

No data migration is provided: greenfield rules apply, and the leftover
table on already-shipped instances is harmless because no code reads or
writes it after this PR.

* docs: sweep remaining references to the per-stack auto-update toggle

The previous commit retired the per-stack Auto-update gate in favor of
configuring auto-update purely through scheduled tasks. This commit
removes the now-stale mentions of that toggle across the operator docs:

- docs/features/sidebar.mdx: drop the Auto-update entry from the Inspect
  group description, the matching screenshot alt-text, and the Skipper
  Note that listed it. Schedule task now carries the cross-link to
  Auto-Update Policies.
- docs/features/stack-management.mdx: drop the Auto-update list item;
  refresh the Schedule task entry to mention the Auto-update Stack action.
- docs/features/dashboard.mdx: rename the Configuration Status row from
  "Auto-update stacks" to "Auto-update schedules" with the new value
  shape, and rewrite the troubleshooting accordion to describe the
  scheduled-tasks invalidation path.
- docs/features/scheduled-operations.mdx: rewrite the Auto-update All
  Stacks row and helper text to reflect that every stack on the node is
  covered (no per-stack opt-out from this surface anymore).
- docs/features/multi-node.mdx: rewrite the Updates column definition to
  derive the Auto/Off flag from enabled Auto-update Stack / Auto-update
  All Stacks schedules instead of the removed per-stack policy.

The auto-update-policies.mdx rewrite in the previous commit already
covered the main reference page. The sidebar-context-menu.png screenshot
will be refreshed on release once the new menu is live in production;
the alt text is updated in this commit so it accurately describes the
shipping state.

No website edits needed: the Auto-Update Policies feature card description
("Schedule automatic image pulls and redeployments per stack on your own
cadence") and the feature matrix labels ("Auto-update stack schedule",
"Auto-update all stacks schedule") remain accurate under the new model.

* fix(stacks): drop orphaned requireAdmin import after auto-update route removal

CI's backend lint step flagged this PR's earlier deletion of the three
/api/stacks/*/auto-update routes: those handlers were the only callers of
`requireAdmin` inside routes/stacks.ts, leaving the named import on line 15
unreferenced. `requirePaid` and `effectiveTier` from the same line are still
in use elsewhere in the file and stay.

tsc --noEmit does not flag unused named imports; ESLint's no-unused-vars
does. Local backend lint reproduces and now reports 0 errors against the
existing 334-warning baseline.

docs/features/scheduled-operations.mdx

@@ -60,7 +60,7 @@ The All tasks toggle swaps the lane track for a sortable table.
 |---|---|---|
 | **Restart Stack** | A specific stack (optionally specific services) on a specific node | Restarts all or selected containers in the stack. |
 | **Auto-update Stack** | A specific stack on a specific node | Checks each image in the stack for a newer tag and recreates the stack if any image has an update. See [Auto-Update Policies](/features/auto-update-policies) for the companion review board. |
-| **Auto-update All Stacks** | A specific node | Runs the auto-update check across every stack on the node that has auto-updates enabled. Stacks with auto-updates turned off are skipped. |
+| **Auto-update All Stacks** | A specific node | Runs the auto-update check across every stack on the node. Pair with **Auto-update Stack** rows when you want different cadences for specific stacks. |
 | **Fleet Snapshot** | The whole fleet | Creates a versioned, fleet-wide snapshot of every node's compose files and `.env` files. See [Fleet Backups](/features/fleet-backups). |
 | **System Prune** | The selected node | Prunes containers, images, networks, and volumes (any subset), optionally filtered by a Docker label. |
 | **Vulnerability Scan** | A specific node | Runs Trivy against every image on the node and persists the results. Requires Trivy to be installed on the target node ([Installing Trivy](/operations/trivy-setup)). |
@@ -88,7 +88,7 @@ Common fields:
 Conditional fields per action:
 
 - **Stack actions** (Restart Stack, Auto-update Stack, Backup Stack Files, Stop / Take Down / Start Stack) add a **Node** combobox and a **Stack** combobox. Restart Stack additionally renders a **Services** checkbox grid sourced from the stack's compose services, so you can scope the restart to a subset instead of restarting the entire stack.
-- **Auto-update All Stacks** adds a **Node** combobox with the helper text "Only stacks with auto-updates enabled on this node will be updated."
+- **Auto-update All Stacks** adds a **Node** combobox with the helper text "Every stack on the selected node will be checked and updated when new images are available."
 - **Vulnerability Scan** adds a **Node** combobox with the helper text "Every image on the selected node will be scanned."
 - **System Prune** adds a **Prune Targets** group (Containers, Images, Networks, Volumes; all selected by default) and a **Label Filter** input for scoping the prune to resources matching a Docker label.