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/sqlite.go

@@ -0,0 +1,31 @@
+package db
+
+import (
+	"database/sql"
+	"fmt"
+	"strings"
+
+	_ "modernc.org/sqlite"
+)
+
+// OpenSQLite opens (creating if needed) a SQLite database tuned for a single
+// writer with concurrent readers: WAL journalling, a busy timeout so writers
+// wait instead of failing, and enforced foreign keys.
+func OpenSQLite(path string) (*sql.DB, error) {
+	if strings.TrimSpace(path) == "" {
+		return nil, fmt.Errorf("sqlite path is required")
+	}
+	dsn := fmt.Sprintf(
+		"file:%s?_pragma=journal_mode(WAL)&_pragma=busy_timeout(5000)&_pragma=foreign_keys(1)&_pragma=synchronous(NORMAL)&_txlock=immediate",
+		path,
+	)
+	db, err := sql.Open("sqlite", dsn)
+	if err != nil {
+		return nil, fmt.Errorf("open sqlite: %w", err)
+	}
+	if err := db.Ping(); err != nil {
+		_ = db.Close()
+		return nil, fmt.Errorf("ping sqlite: %w", err)
+	}
+	return db, nil
+}