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/notifications/repository.go

@@ -0,0 +1,14 @@
+package notifications
+
+import (
+	"context"
+
+	"shared-album/internal/models"
+)
+
+// Repository is the persistence contract for the notifications domain.
+type Repository interface {
+	Create(ctx context.Context, n *models.Notification) error
+	List(ctx context.Context, userID string, limit int) ([]models.Notification, error)
+	MarkRead(ctx context.Context, id, userID string) error
+}