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/photos/sqlite_helpers.go

@@ -0,0 +1,30 @@
+package photos
+
+import (
+	"time"
+
+	"shared-album/internal/sqliteutil"
+)
+
+func nullIfEmpty(s string) any {
+	if s == "" {
+		return nil
+	}
+	return s
+}
+
+func nullTime(t *time.Time) any {
+	if t == nil {
+		return nil
+	}
+	return sqliteutil.FormatTime(*t)
+}
+
+func nullFloat(f *float64) any {
+	if f == nil {
+		return nil
+	}
+	return *f
+}
+
+func nowUTC() time.Time { return time.Now().UTC() }