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/social/sqlite_store.go

@@ -0,0 +1,101 @@
+package social
+
+import (
+	"context"
+	"database/sql"
+	"errors"
+
+	"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) Like(ctx context.Context, photoID, userID string) error {
+	_, err := s.db.ExecContext(ctx,
+		`INSERT INTO likes (photo_id, user_id) VALUES ($1, $2)
+		 ON CONFLICT (photo_id, user_id) DO NOTHING`, photoID, userID)
+	return err
+}
+
+func (s *SQLiteStore) Unlike(ctx context.Context, photoID, userID string) error {
+	_, err := s.db.ExecContext(ctx,
+		`DELETE FROM likes WHERE photo_id = $1 AND user_id = $2`, photoID, userID)
+	return err
+}
+
+func (s *SQLiteStore) CreateComment(ctx context.Context, c *models.Comment) error {
+	var createdAt, updatedAt string
+	err := s.db.QueryRowContext(ctx,
+		`INSERT INTO comments (id, photo_id, user_id, text)
+		 VALUES ($1, $2, $3, $4)
+		 RETURNING created_at, updated_at`,
+		c.ID, c.PhotoID, c.UserID, c.Text,
+	).Scan(&createdAt, &updatedAt)
+	if err != nil {
+		return err
+	}
+	c.CreatedAt = sqliteutil.ParseTime(createdAt)
+	c.UpdatedAt = sqliteutil.ParseTime(updatedAt)
+	return nil
+}
+
+func (s *SQLiteStore) ListComments(ctx context.Context, photoID string) ([]models.Comment, error) {
+	rows, err := s.db.QueryContext(ctx,
+		`SELECT c.id, c.photo_id, c.user_id, c.text, c.created_at, c.updated_at,
+		        u.id, u.name, COALESCE(u.avatar_url, '')
+		 FROM comments c
+		 JOIN users u ON u.id = c.user_id
+		 WHERE c.photo_id = $1
+		 ORDER BY c.created_at`, photoID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	out := []models.Comment{}
+	for rows.Next() {
+		var c models.Comment
+		var createdAt, updatedAt string
+		if err := rows.Scan(&c.ID, &c.PhotoID, &c.UserID, &c.Text, &createdAt, &updatedAt,
+			&c.User.ID, &c.User.Name, &c.User.AvatarURL); err != nil {
+			return nil, err
+		}
+		c.CreatedAt = sqliteutil.ParseTime(createdAt)
+		c.UpdatedAt = sqliteutil.ParseTime(updatedAt)
+		out = append(out, c)
+	}
+	return out, rows.Err()
+}
+
+func (s *SQLiteStore) GetComment(ctx context.Context, id string) (*models.Comment, error) {
+	var c models.Comment
+	var createdAt, updatedAt string
+	err := s.db.QueryRowContext(ctx,
+		`SELECT id, photo_id, user_id, text, created_at, updated_at FROM comments WHERE id = $1`, id).
+		Scan(&c.ID, &c.PhotoID, &c.UserID, &c.Text, &createdAt, &updatedAt)
+	if err != nil {
+		if errors.Is(err, sql.ErrNoRows) {
+			return nil, ErrNotFound
+		}
+		return nil, err
+	}
+	c.CreatedAt = sqliteutil.ParseTime(createdAt)
+	c.UpdatedAt = sqliteutil.ParseTime(updatedAt)
+	return &c, nil
+}
+
+func (s *SQLiteStore) DeleteComment(ctx context.Context, id string) error {
+	res, err := s.db.ExecContext(ctx, `DELETE FROM comments WHERE id = $1`, id)
+	if err != nil {
+		return err
+	}
+	if n, _ := res.RowsAffected(); n == 0 {
+		return ErrNotFound
+	}
+	return nil
+}