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/notifications/sqlite_store.go

@@ -0,0 +1,88 @@
+package notifications
+
+import (
+	"context"
+	"database/sql"
+	"encoding/json"
+	"errors"
+	"time"
+
+	"shared-album/internal/models"
+	"shared-album/internal/sqliteutil"
+)
+
+// SQLiteStore is the SQLite implementation of Repository.
+type SQLiteStore struct {
+	db *sql.DB
+}
+
+func NewSQLiteStore(db *sql.DB) *SQLiteStore { return &SQLiteStore{db: db} }
+
+func (s *SQLiteStore) Create(ctx context.Context, n *models.Notification) error {
+	payload, err := json.Marshal(n.PayloadJSON)
+	if err != nil {
+		return err
+	}
+	var createdAt string
+	err = s.db.QueryRowContext(ctx,
+		`INSERT INTO notifications (id, user_id, type, payload_json)
+		 VALUES ($1, $2, $3, $4)
+		 RETURNING created_at`,
+		n.ID, n.UserID, n.Type, string(payload)).Scan(&createdAt)
+	if err != nil {
+		return err
+	}
+	n.CreatedAt = sqliteutil.ParseTime(createdAt)
+	return nil
+}
+
+func (s *SQLiteStore) List(ctx context.Context, userID string, limit int) ([]models.Notification, error) {
+	if limit <= 0 || limit > 200 {
+		limit = 100
+	}
+	rows, err := s.db.QueryContext(ctx,
+		`SELECT id, user_id, type, payload_json, read_at, created_at
+		 FROM notifications
+		 WHERE user_id = $1
+		 ORDER BY created_at DESC
+		 LIMIT $2`, userID, limit)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	out := []models.Notification{}
+	for rows.Next() {
+		var n models.Notification
+		var payload, createdAt string
+		var readAt sql.NullString
+		if err := rows.Scan(&n.ID, &n.UserID, &n.Type, &payload, &readAt, &createdAt); err != nil {
+			return nil, err
+		}
+		_ = json.Unmarshal([]byte(payload), &n.PayloadJSON)
+		n.ReadAt = sqliteutil.ParseNullTime(readAt)
+		n.CreatedAt = sqliteutil.ParseTime(createdAt)
+		out = append(out, n)
+	}
+	return out, rows.Err()
+}
+
+func (s *SQLiteStore) MarkRead(ctx context.Context, id, userID string) error {
+	res, err := s.db.ExecContext(ctx,
+		`UPDATE notifications SET read_at = $3 WHERE id = $1 AND user_id = $2 AND read_at IS NULL`,
+		id, userID, sqliteutil.FormatTime(time.Now()))
+	if err != nil {
+		return err
+	}
+	if n, _ := res.RowsAffected(); n == 0 {
+		var exists int
+		err := s.db.QueryRowContext(ctx,
+			`SELECT 1 FROM notifications WHERE id = $1 AND user_id = $2`, id, userID).Scan(&exists)
+		if err != nil {
+			if errors.Is(err, sql.ErrNoRows) {
+				return ErrNotFound
+			}
+			return err
+		}
+	}
+	return nil
+}