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/store/store.go

@@ -0,0 +1,55 @@
+// Package store builds the per-domain metadata repositories for a chosen
+// backend. Domains depend on their Repository interfaces; this factory is the
+// single place that knows the concrete implementations.
+package store
+
+import (
+	"database/sql"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+
+	"shared-album/internal/albums"
+	"shared-album/internal/auth"
+	"shared-album/internal/members"
+	"shared-album/internal/notifications"
+	"shared-album/internal/photos"
+	"shared-album/internal/sharelinks"
+	"shared-album/internal/social"
+)
+
+// Repos bundles one repository per domain.
+type Repos struct {
+	Auth          auth.Repository
+	Albums        albums.Repository
+	Members       members.Repository
+	Photos        photos.Repository
+	Social        social.Repository
+	Notifications notifications.Repository
+	ShareLinks    sharelinks.Repository
+}
+
+// NewPostgres builds the repositories backed by PostgreSQL.
+func NewPostgres(pool *pgxpool.Pool) Repos {
+	return 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),
+	}
+}
+
+// NewSQLite builds the repositories backed by SQLite.
+func NewSQLite(db *sql.DB) Repos {
+	return Repos{
+		Auth:          auth.NewSQLiteStore(db),
+		Albums:        albums.NewSQLiteStore(db),
+		Members:       members.NewSQLiteStore(db),
+		Photos:        photos.NewSQLiteStore(db),
+		Social:        social.NewSQLiteStore(db),
+		Notifications: notifications.NewSQLiteStore(db),
+		ShareLinks:    sharelinks.NewSQLiteStore(db),
+	}
+}