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>

internal/library/scanrun.go

@@ -245,6 +245,7 @@ func RunScan(
 	}
 
 	logger.Info("scan run complete", "id", row.ID, "error", errMsg)
+	notifyScanFinished(ctx, errMsg)
 	publishScanEvent("scan.run_finished", row.ID, map[string]any{
 		"error_message": errMsg,
 	})