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/service.go

@@ -22,13 +22,13 @@ type AlbumAccess interface {
 }
 
 type Service struct {
-	store   *Store
+	store   Repository
 	albums  AlbumAccess
 	storage storage.Backend
 	hub     *sse.Hub
 }
 
-func NewService(store *Store, albums AlbumAccess, storage storage.Backend, hub *sse.Hub) *Service {
+func NewService(store Repository, albums AlbumAccess, storage storage.Backend, hub *sse.Hub) *Service {
 	return &Service{store: store, albums: albums, storage: storage, hub: hub}
 }