feat(notifications): library health reaches admins, coalesced (#5341)

- A failed scan run sends scan_failed. Each failure adds to the count and
  the notice shows the latest error. A scan cut short by shutdown says
  nothing.
- Marking tracks missing sends tracks_missing with a running count.
- A duplicate sweep that proposes a group it had not proposed before
  sends duplicates_found, counting everything awaiting review. A sweep
  that only re-finds known groups stays quiet, so a read notice isn't
  repeated every sweep (CountDuplicateGroupsDetectedSince).
- A playback-error report sends playback_errors, counting the unresolved
  errors (CountUnresolvedPlaybackErrors).

The library package gets its notifier as a package-level SetNotifier
beside SetEventBus, for the same reason the bus is package-level.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/minstrel/main.go

@@ -321,6 +321,7 @@ func run() error {
 
 	lidarrReconciler := lidarrrequests.NewReconciler(pool, lidarrCfg, lidarrClientFn, logger.With("component", "lidarr"), bus)
 	lidarrReconciler.SetNotifier(notifier)
+	library.SetNotifier(notifier)
 	go lidarrReconciler.Run(ctx)
 
 	// Missing-file re-acquisition (milestone #290). Turns albums whose files