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/app/share_flow_test.go

@@ -27,6 +27,7 @@ import (
 	"shared-album/internal/sharelinks"
 	"shared-album/internal/sse"
 	"shared-album/internal/storage"
+	"shared-album/internal/store"
 )
 
 func integrationPool(t *testing.T) *pgxpool.Pool {
@@ -70,7 +71,7 @@ func runShareFlow(t *testing.T, backend storage.Backend, verify func(t *testing.
 	gin.SetMode(gin.TestMode)
 
 	hub := sse.NewHub(1)
-	srv := New(&config.Config{}, pool, backend, hub)
+	srv := New(&config.Config{}, store.NewPostgres(pool), pool.Ping, backend, hub)
 	router := srv.Router()
 
 	authStore := auth.NewStore(pool)