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/notify.go

@@ -0,0 +1,44 @@
+package library
+
+import (
+	"context"
+	"sync"
+
+	"github.com/jackc/pgx/v5/pgtype"
+
+	"git.fabledsword.com/bvandeusen/minstrel/internal/notifications"
+)
+
+// Package-level notifier for library-health notices to admins (M489): a
+// scan that failed, tracks gone missing, new duplicates. Set once at startup
+// beside SetEventBus, for the same reason the bus is package-level. Nil, as in
+// tests that never set it, means nobody is told.
+var (
+	notifierMu sync.RWMutex
+	notifier   *notifications.Notifier
+)
+
+// SetNotifier wires the notifications inbox into the library package.
+func SetNotifier(n *notifications.Notifier) {
+	notifierMu.Lock()
+	defer notifierMu.Unlock()
+	notifier = n
+}
+
+// notifyAdmins tells every admin about a library-health event. These kinds
+// coalesce, so a burst is one unread notice with a running count.
+func notifyAdmins(ctx context.Context, kind notifications.Kind, p notifications.Payload) {
+	notifierMu.RLock()
+	n := notifier
+	notifierMu.RUnlock()
+	n.NotifyLogged(ctx, kind, notifications.ToAdmins(pgtype.UUID{}), p.Map())
+}
+
+// notifyScanFinished tells admins a scan run ended in error. A scan cut short
+// by shutdown is not a failure anyone needs telling about.
+func notifyScanFinished(ctx context.Context, errMsg string) {
+	if errMsg == "" || ctx.Err() != nil {
+		return
+	}
+	notifyAdmins(ctx, notifications.KindScanFailed, notifications.Payload{Count: 1, Detail: errMsg})
+}