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/migrations/sqlite/000001_init.down.sql

@@ -0,0 +1,15 @@
+DROP TABLE IF EXISTS variant_jobs;
+DROP TABLE IF EXISTS photo_objects;
+DROP TABLE IF EXISTS objects;
+DROP TABLE IF EXISTS photo_tags;
+DROP TABLE IF EXISTS tags;
+DROP TABLE IF EXISTS refresh_tokens;
+DROP TABLE IF EXISTS share_links;
+DROP TABLE IF EXISTS notifications;
+DROP TABLE IF EXISTS likes;
+DROP TABLE IF EXISTS comments;
+DROP TABLE IF EXISTS invites;
+DROP TABLE IF EXISTS album_members;
+DROP TABLE IF EXISTS photos;
+DROP TABLE IF EXISTS albums;
+DROP TABLE IF EXISTS users;