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/db/migrate.go

@@ -8,8 +8,10 @@ import (
 	"database/sql"
 	"github.com/golang-migrate/migrate/v4"
 	"github.com/golang-migrate/migrate/v4/database/pgx/v5"
+	sqlitemig "github.com/golang-migrate/migrate/v4/database/sqlite"
 	"github.com/golang-migrate/migrate/v4/source/iofs"
 	_ "github.com/jackc/pgx/v5/stdlib"
+	_ "modernc.org/sqlite"
 )
 
 // Migrate applies all up migrations from the given fs (root contains *.sql files).
@@ -39,3 +41,32 @@ func Migrate(dsn string, migrations fs.FS) error {
 	}
 	return nil
 }
+
+// MigrateSQLite applies all up migrations from the given fs to a SQLite database
+// at path (creating it if needed).
+func MigrateSQLite(path string, migrations fs.FS) error {
+	source, err := iofs.New(migrations, ".")
+	if err != nil {
+		return fmt.Errorf("migrations source: %w", err)
+	}
+
+	sqlDB, err := sql.Open("sqlite", path)
+	if err != nil {
+		return fmt.Errorf("open sql db: %w", err)
+	}
+	defer sqlDB.Close()
+
+	driver, err := sqlitemig.WithInstance(sqlDB, &sqlitemig.Config{})
+	if err != nil {
+		return fmt.Errorf("migrate driver: %w", err)
+	}
+
+	m, err := migrate.NewWithInstance("iofs", source, "sqlite", driver)
+	if err != nil {
+		return fmt.Errorf("migrate instance: %w", err)
+	}
+	if err := m.Up(); err != nil && !errors.Is(err, migrate.ErrNoChange) {
+		return fmt.Errorf("migrate up: %w", err)
+	}
+	return nil
+}