feat(backend): immutable cache dir scheme + stale sweep

backend/internal/store/store.go

@@ -262,6 +262,24 @@ func (s *Store) ListBooks(ctx context.Context, libID int64, q, prefix string, us
 	return out, rows.Err()
 }
 
+func (s *Store) BookHashes(ctx context.Context) (map[int64][2]int64, error) {
+	rows, err := s.P.Query(ctx, "SELECT id, file_size, mod_ts FROM books")
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	out := map[int64][2]int64{}
+	for rows.Next() {
+		var id int64
+		var v [2]int64
+		if err := rows.Scan(&id, &v[0], &v[1]); err != nil {
+			return nil, err
+		}
+		out[id] = v
+	}
+	return out, rows.Err()
+}
+
 // ---------- progress ----------
 
 func (s *Store) UpsertProgress(ctx context.Context, userID, libID int64, bookPath string, locator []byte, percent float64) error {