feat: health-gated updates and rollback readiness (#1354)

* feat: classify stack deploy and update failures with suggested next actions

Failed deploy and update responses now carry a failure classification
(cause category, headline, and suggested next step) derived from the
compose error output. The recovery panel and chip render the
classification and include it in copied diagnostics, and gateway-style
failures surface as a node-unreachable cause.

* feat: add update and rollback readiness reports for stacks

Before a manual update, Sencho now shows an advisory readiness verdict
computed from the stored preflight result, open drift findings, live
container health, the pending image change, the rollback backup slot,
and node disk headroom. The Stack Dossier gains a rollback readiness
section that states what a rollback can restore and explicitly
discloses that volume and bind-mounted data are not covered. Toolbar
and sidebar updates now share one update path, and admins can create a
fleet snapshot from the readiness dialog before updating. Nodes that do
not advertise the capability keep the direct update flow.

* feat: observe stack health after updates with a post-deploy health gate

After a deploy or update succeeds, Sencho now watches the stack for a
configurable observation window and records a passed, failed, or
unknown verdict: containers must stay running, healthchecks must report
healthy, and restart loops or disappearing containers fail the gate.
The deploy panel shows the observation live and holds off auto-closing
until the verdict lands, a failed gate surfaces the existing recovery
actions including rollback, and the stack timeline records update
started and gate verdict events. Scheduled, webhook, bulk, and
git-source updates are gated the same way; rollbacks and installs are
deliberately not. The gate is observational only and can be tuned or
disabled per node under host alert settings.

* docs: document health-gated updates and rollback readiness

New operator page covering the update readiness dialog, the post-update
health gate and its settings, the rollback readiness disclosure, and
classified failures, with cross-links from the atomic deployments and
deploy progress pages. The API reference gains the readiness and
health-gate endpoints, the healthGateId success field, and the failure
classification schema on deploy and update error responses.

* feat: withhold the success verdict while the health gate observes

An update used to show a green Succeeded that a failed health gate then
contradicted moments later. The deploy modal now reports Verifying
health while the gate observes, shows success only when the gate
passes, and makes a failed or unknown gate the headline result; success
toasts soften to a verifying message while a gate runs. The mobile
recovery card groups its actions behind one bottom-right Take action
menu so it stays compact on a phone, with the classified cause still
visible on the card. A successful image update now also counts as the
last known-good marker in rollback readiness, and the docs gain
screenshots of the readiness dialog, gate states, dossier section, and
settings.

* fix: harden log format strings and the env existence path check

Log calls that interpolated the stack name into the console format
string now use constant format strings with placeholder arguments, and
envExists validates path containment inline at its filesystem access,
matching the established patterns used elsewhere in the same files.

* test: adapt deploy modal success specs to the post-deploy health gate

The deploy feedback modal now withholds its success verdict while the
health gate observes the new containers, showing "Verifying health"
until the gate passes. The two success-path E2E tests waited for
"Succeeded" within the gate's 90s default window and timed out.

Shorten the observation window to the 15s minimum for these tests via
the settings API, assert the verify-then-succeed sequence the modal
actually renders, and restore the default window afterward so the test
value does not leak into later runs.

* fix: serialize health gate polling and harden gate observation

Address race conditions in the post-update health gate found in review.

Backend: the gate poller used setInterval, so a Docker observe slower
than the 5s tick could overlap the next poll and corrupt the restart and
missing-container accounting, and a wedged socket could leave a poll
pending forever. Polling is now single-flight: each cycle self-schedules
the next only after it settles, and the observe is bounded by an 8s
timeout so a hung probe counts as a poll error and resolves the gate
unknown after three in a row.

Frontend: the gate poller could overlap requests, letting a slow earlier
"observing" response overwrite an already-applied terminal verdict. It is
now single-flight with a terminal latch, so a late response can never
roll the UI back from passed or failed.

Also reject a non-digit nodeId on the snapshot coverage route instead of
letting parseInt coerce it, document that turning off the deploy progress
panel opts out of the live gate UI while the gate still runs server-side,
and add gate-coverage tests for the webhook, git source, and auto-update
apply paths plus the new single-flight, observe-timeout, and recovery
cases.

docs/features/atomic-deployments.mdx

@@ -5,7 +5,7 @@ description: Wrap every deploy and update in a backup, a 3-second health probe,
 
 Sencho wraps every protected deploy in a four-step safety net: it copies the current `compose.yaml` and `.env` to a writable backup directory, runs the requested compose action, waits 3 seconds for the new containers to settle, then checks them for a non-zero exit code. If any container has crashed, Sencho restores the backed-up files and re-deploys automatically.
 
-The same backup also powers the **Rollback** action in the stack editor, so you can roll a stack back to its last good configuration on demand.
+The same backup also powers the **Rollback** action in the stack editor, so you can roll a stack back to its last good configuration on demand. To see in advance whether that rollback would actually help, and to watch container health for longer than the 3-second probe, see [Health-Gated Updates](/features/health-gated-updates).
 
 ## How it works
 
@@ -54,7 +54,7 @@ A restore is a faithful revert, not an overlay. Sencho replaces the compose file
     A user without the `stack:deploy` permission will still see the menu entry; the rejection comes from the backend with a permission error after they click. Ask an admin to grant `stack:deploy` through **Settings · Roles & Access** if that happens.
   </Accordion>
   <Accordion title="The deploy succeeded but a service crashed seconds later">
-    The health probe is a 3-second window after `docker compose up -d` returns. Crashes that happen after that window are out of scope for atomic rollback by design, because Sencho cannot tell whether a late exit is a real failure or a normal restart. For ongoing health, use **Auto-Heal Policies** to restart unhealthy containers automatically and **Alert Rules** to page you when a container exits unexpectedly.
+    The health probe is a 3-second window after `docker compose up -d` returns. Crashes that happen after that window are out of scope for atomic rollback by design, because Sencho cannot tell whether a late exit is a real failure or a normal restart. The [health gate](/features/health-gated-updates) covers exactly this period: it observes the stack for a configurable window after the update, records a verdict on the stack timeline, and offers a manual rollback when containers do not stay healthy. For ongoing health beyond that, use **Auto-Heal Policies** to restart unhealthy containers automatically and **Alert Rules** to page you when a container exits unexpectedly.
   </Accordion>
   <Accordion title="The deploy progress modal showed 'Rollback failed - manual intervention may be required'">
     This message means the auto-rollback attempted to restore the backup and re-deploy, but the restore step or the re-deploy itself errored out. The backup files are still at `<DATA_DIR>/backups/<stack>/`. To recover: