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/db/queries/duplicates.sql

@@ -154,3 +154,13 @@ SELECT p.id                  AS group_id,
 UPDATE duplicate_groups
    SET status = 'dismissed', resolved_at = now()
  WHERE id = sqlc.arg(id) AND status = 'pending';
+
+-- name: CountDuplicateGroupsDetectedSince :one
+-- Pending proposals first made at or after `since`: what a sweep that started
+-- then found for the first time. A refreshed proposal keeps its detected_at,
+-- so a sweep that only re-finds known groups counts none (M489: admins are
+-- told about new duplicates, not reminded of the same ones every sweep).
+SELECT count(*)::bigint
+  FROM duplicate_groups
+ WHERE status = 'pending'
+   AND detected_at >= sqlc.arg(since);

internal/db/queries/playback_errors.sql

@@ -61,3 +61,6 @@ UPDATE playback_errors
        resolved_by = $2,
        resolution  = $3
  WHERE track_id = $1 AND resolved_at IS NULL;
+
+-- name: CountUnresolvedPlaybackErrors :one
+SELECT count(*)::bigint FROM playback_errors WHERE resolved_at IS NULL;