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/duplicate_sweep.go

@@ -14,6 +14,7 @@ import (
 	"github.com/jackc/pgx/v5/pgxpool"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db/dbq"
+	"git.fabledsword.com/bvandeusen/minstrel/internal/notifications"
 	syncpkg "git.fabledsword.com/bvandeusen/minstrel/internal/sync"
 )
 
@@ -101,12 +102,37 @@ func runDuplicateSweep(
 		}
 	}
 
+	if runErr == nil {
+		notifyNewDuplicates(finishCtx, q, sweep.StartedAt, logger)
+	}
+
 	logger.Info("duplicate sweep complete",
 		"candidates", res.Candidates, "groups", res.Groups, "proposed", res.Proposed,
 		"suppressed", res.Suppressed, "retired", res.Retired, "oversize", res.Oversize, "err", runErr)
 	return res, runErr
 }
 
+// notifyNewDuplicates tells admins when a sweep proposed a group it had not
+// proposed before (M489), counting every proposal awaiting review. A sweep
+// that only re-finds known groups says nothing, so reading the notice once
+// is enough until something new turns up.
+func notifyNewDuplicates(ctx context.Context, q *dbq.Queries, sweepStarted pgtype.Timestamptz, logger *slog.Logger) {
+	fresh, err := q.CountDuplicateGroupsDetectedSince(ctx, sweepStarted)
+	if err != nil {
+		logger.Warn("duplicate sweep: counting new proposals failed", "err", err)
+		return
+	}
+	if fresh == 0 {
+		return
+	}
+	pending, err := q.CountPendingDuplicateGroups(ctx)
+	if err != nil {
+		logger.Warn("duplicate sweep: counting pending proposals failed", "err", err)
+		return
+	}
+	notifyAdmins(ctx, notifications.KindDuplicatesFound, notifications.Payload{Count: pending})
+}
+
 func sweepDuplicates(
 	ctx context.Context, q *dbq.Queries, sweepID pgtype.UUID, cfg FingerprintSettings, pageSize int32,
 ) (DuplicateSweepResult, error) {