fix: distinguish failed image-update checks from "up to date" (#1470)

* fix: distinguish failed image-update checks from "up to date"

The image-update detector collapsed every failure (registry unreachable,
missing auth, rate limit, unresolved local digest) into hasUpdate:false and
dropped the captured reason, so a failed check was indistinguishable from a
current image and never raised a notification, even while a manual stack
update still pulled a newer image.

Detection now records a tri-state per stack (ok / partial / failed) with the
failure reason, exposed via a new GET /api/image-updates/detail (the boolean
GET / is unchanged so fleet aggregation is unaffected). A fully-failed check
preserves the last known has_update, so a transient outage neither erases a
real update nor flaps the notification state. The sidebar shows a muted
"couldn't check" indicator with the reason on hover, and the Update board
lists stacks whose check failed in a "could not be checked" advisory.

Detector hardening: the manifest digest lookup issues HEAD first (falling back
to GET) so it no longer draws down Docker Hub's anonymous pull-rate budget, and
local RepoDigest matching is normalized so official library/* images resolve
their digest instead of falling through to a silent "no update".

* fix: preserve confirmed updates through partial checks; tighten failure surfacing

Address review findings on the tri-state image-update detection:

- A partial check (some images errored) no longer erases a previously
  confirmed update; only a fully-ok check can lower has_update, so a single
  image's registry blip cannot drop the stack's update and re-fire the
  notification on recovery. Adds a regression test.
- The image-level catch stores getErrorMessage(e) rather than raw String(e),
  since that value surfaces verbatim in the sidebar tooltip and readiness
  advisory.
- useImageUpdates and the readiness detail fetch now log unexpected non-ok
  responses instead of silently leaving stale state.
- Remove an unused checkFailedCount derivation (the row indicator is driven by
  the checkStatus prop).
- Reword the recordStackCheckFailure docstring and the HEAD-first comment.

backend/src/routes/imageUpdates.ts

@@ -37,6 +37,19 @@ imageUpdatesRouter.get('/', authMiddleware, (req: Request, res: Response): void
   }
 });
 
+// Rich per-stack status (hasUpdate + check outcome + reason) for the sidebar and
+// readiness view. Auth-only, matching GET /; the boolean GET / is left intact so
+// the cross-version fleet aggregation contract is unaffected.
+imageUpdatesRouter.get('/detail', authMiddleware, (req: Request, res: Response): void => {
+  try {
+    const nodeId = req.nodeId ?? NodeRegistry.getInstance().getDefaultNodeId();
+    res.json(DatabaseService.getInstance().getStackUpdateDetail(nodeId));
+  } catch (error) {
+    console.error('Failed to fetch image update detail:', error);
+    res.status(500).json({ error: 'Failed to fetch image update detail' });
+  }
+});
+
 imageUpdatesRouter.post('/refresh', authMiddleware, (req: Request, res: Response): void => {
   if (!requireAdmin(req, res)) return;
   try {