feat(metadata): SQLite backend, storage reconciliation and local defaults

- pluggable per-domain Repository interfaces; PostgreSQL and SQLite impls
- SQLite on modernc.org/sqlite (WAL, busy_timeout, foreign_keys, immediate tx)
- separate migrations/sqlite and db.MigrateSQLite; store.NewPostgres/NewSQLite
- cmd/reconcile compares object storage with the journal (report, then --delete)
- storage.Backend.List and photos.Repository.AllStorageKeys
- DB_BACKEND/SQLITE_PATH and STORAGE_BACKEND/FS_ROOT selectable; local defaults
  (sqlite ./galo.db, fs ./data)
- Makefile reconcile/reconcile-delete; docs and .env.example updated

backend/internal/photos/objects.go

@@ -368,3 +368,21 @@ func (s *Store) FailVariantJob(ctx context.Context, photoID, errMsg string, maxA
 	}
 	return tx.Commit(ctx)
 }
+
+// AllStorageKeys returns every key registered in the object journal.
+func (s *Store) AllStorageKeys(ctx context.Context) ([]string, error) {
+	rows, err := s.pool.Query(ctx, `SELECT storage_key FROM objects`)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	var keys []string
+	for rows.Next() {
+		var k string
+		if err := rows.Scan(&k); err != nil {
+			return nil, err
+		}
+		keys = append(keys, k)
+	}
+	return keys, rows.Err()
+}