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_sqlite_test.go

@@ -0,0 +1,37 @@
+package db
+
+import (
+	"io/fs"
+	"path/filepath"
+	"testing"
+
+	assets "shared-album"
+)
+
+func TestMigrateSQLite(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "meta.db")
+	migFS, err := fs.Sub(assets.Migrations, "migrations/sqlite")
+	if err != nil {
+		t.Fatalf("migrations fs: %v", err)
+	}
+	if err := MigrateSQLite(path, migFS); err != nil {
+		t.Fatalf("migrate: %v", err)
+	}
+	if err := MigrateSQLite(path, migFS); err != nil {
+		t.Fatalf("migrate again: %v", err)
+	}
+
+	db, err := OpenSQLite(path)
+	if err != nil {
+		t.Fatalf("open: %v", err)
+	}
+	defer db.Close()
+
+	var n int
+	if err := db.QueryRow(`SELECT count(*) FROM sqlite_master WHERE type='table' AND name='objects'`).Scan(&n); err != nil {
+		t.Fatalf("query: %v", err)
+	}
+	if n != 1 {
+		t.Fatalf("objects table missing after migration")
+	}
+}