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/repo/contract/postgres_test.go

@@ -0,0 +1,78 @@
+package contract
+
+import (
+	"context"
+	"fmt"
+	"io/fs"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	assets "shared-album"
+	"shared-album/internal/albums"
+	"shared-album/internal/auth"
+	"shared-album/internal/db"
+	"shared-album/internal/members"
+	"shared-album/internal/notifications"
+	"shared-album/internal/photos"
+	"shared-album/internal/sharelinks"
+	"shared-album/internal/social"
+)
+
+// TestPostgresContract runs the shared suite against PostgreSQL inside its own
+// schema, so it never clashes with other packages that use the same database.
+func TestPostgresContract(t *testing.T) {
+	base := os.Getenv("TEST_DATABASE_URL")
+	if base == "" {
+		t.Skip("TEST_DATABASE_URL not set")
+	}
+	ctx := context.Background()
+
+	admin, err := pgxpool.New(ctx, base)
+	if err != nil {
+		t.Skipf("cannot connect: %v", err)
+	}
+	if err := admin.Ping(ctx); err != nil {
+		t.Skipf("cannot ping: %v", err)
+	}
+	t.Cleanup(admin.Close)
+
+	schema := fmt.Sprintf("ct_%d", time.Now().UnixNano())
+	if _, err := admin.Exec(ctx, "CREATE SCHEMA "+schema); err != nil {
+		t.Fatalf("create schema: %v", err)
+	}
+	t.Cleanup(func() {
+		_, _ = admin.Exec(context.Background(), "DROP SCHEMA "+schema+" CASCADE")
+	})
+
+	dsn := base + "&options=-c%20search_path%3D" + schema
+	migFS, err := fs.Sub(assets.Migrations, "migrations")
+	if err != nil {
+		t.Fatalf("migrations fs: %v", err)
+	}
+	if err := db.Migrate(dsn, migFS); err != nil {
+		t.Fatalf("migrate: %v", err)
+	}
+
+	pool, err := pgxpool.New(ctx, dsn)
+	if err != nil {
+		t.Fatalf("pool: %v", err)
+	}
+	t.Cleanup(pool.Close)
+
+	Run(t, Repos{
+		Auth:          auth.NewStore(pool),
+		Albums:        albums.NewStore(pool),
+		Members:       members.NewStore(pool),
+		Photos:        photos.NewStore(pool),
+		Social:        social.NewStore(pool),
+		Notifications: notifications.NewStore(pool),
+		ShareLinks:    sharelinks.NewStore(pool),
+		Reset: func(ctx context.Context) error {
+			_, err := pool.Exec(ctx, `TRUNCATE users, objects CASCADE`)
+			return err
+		},
+	})
+}