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/embed.go

@@ -7,7 +7,7 @@ import "embed"
 //go:embed all:static
 var Static embed.FS
 
-// Migrations holds the SQL migration files.
+// Migrations holds the SQL migration files for every supported backend.
 //
-//go:embed migrations/*.sql
+//go:embed migrations/*.sql migrations/sqlite/*.sql
 var Migrations embed.FS