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/sqlite_test.go

@@ -0,0 +1,57 @@
+package contract
+
+import (
+	"context"
+	"io/fs"
+	"path/filepath"
+	"testing"
+
+	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"
+)
+
+func TestSQLiteContract(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 := db.MigrateSQLite(path, migFS); err != nil {
+		t.Fatalf("migrate: %v", err)
+	}
+	database, err := db.OpenSQLite(path)
+	if err != nil {
+		t.Fatalf("open: %v", err)
+	}
+	t.Cleanup(func() { database.Close() })
+
+	Run(t, Repos{
+		Auth:          auth.NewSQLiteStore(database),
+		Albums:        albums.NewSQLiteStore(database),
+		Members:       members.NewSQLiteStore(database),
+		Photos:        photos.NewSQLiteStore(database),
+		Social:        social.NewSQLiteStore(database),
+		Notifications: notifications.NewSQLiteStore(database),
+		ShareLinks:    sharelinks.NewSQLiteStore(database),
+		Reset: func(ctx context.Context) error {
+			tables := []string{
+				"variant_jobs", "photo_objects", "objects", "photo_tags", "tags",
+				"refresh_tokens", "share_links", "notifications", "likes", "comments",
+				"invites", "album_members", "photos", "albums", "users",
+			}
+			for _, tbl := range tables {
+				if _, err := database.ExecContext(ctx, "DELETE FROM "+tbl); err != nil {
+					return err
+				}
+			}
+			return nil
+		},
+	})
+}