fix(lidarrquarantine): translate FK violation to ErrTrackNotFound + drop unneeded *string AlbumTitle

- Flag now detects Postgres SQLSTATE 23503 (foreign_key_violation) on
  UpsertQuarantine and surfaces ErrTrackNotFound instead of a wrapped
  generic. T8's handler can now return 404 track_not_found cleanly.
  Constraint-name guard ('track' substring) keeps a future user_id FK from
  mis-mapping to the same error.
- AdminQueueRow.AlbumTitle is now string (not *string). albums.title is
  NOT NULL upstream; the empty-string-to-nil dance was answering a
  nullability question that doesn't exist in the schema.
- Add TestFlag_NonexistentTrackReturnsErrTrackNotFound covering the new
  branch.

internal/lidarrquarantine/service_test.go

@@ -9,6 +9,7 @@ import (
 	"path/filepath"
 	"testing"
 
+	"github.com/jackc/pgx/v5/pgtype"
 	"github.com/jackc/pgx/v5/pgxpool"
 
 	"git.fabledsword.com/bvandeusen/minstrel/internal/db"
@@ -117,6 +118,21 @@ func TestFlag_UpsertOnSecondFlag(t *testing.T) {
 	}
 }
 
+func TestFlag_NonexistentTrackReturnsErrTrackNotFound(t *testing.T) {
+	pool := newPool(t)
+	user := seedUser(t, pool, "alice")
+
+	var bogus pgtype.UUID
+	bogus.Bytes = [16]byte{1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16}
+	bogus.Valid = true
+
+	svc := NewService(pool, lidarrconfig.New(pool), nil)
+	_, err := svc.Flag(context.Background(), user.ID, bogus, "bad_rip", "")
+	if !errors.Is(err, ErrTrackNotFound) {
+		t.Errorf("err = %v, want ErrTrackNotFound", err)
+	}
+}
+
 func TestFlag_BadReasonRejected(t *testing.T) {
 	pool := newPool(t)
 	user := seedUser(t, pool, "alice")