fix: cast week_start to text in SELECT and add error logging for silent 500

pgx v5 cannot scan a PostgreSQL DATE column directly into a Go string.
The WHERE clause already used week_start::text but the SELECT did not,
causing GetByWeek to return a scan error that the GenerateMenu handler
swallowed without logging (just returned 500).

- repository.go: SELECT mp.week_start::text instead of mp.week_start
- handler.go: add slog.Error before the silent 500 branch

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/menu/repository.go

@@ -28,7 +28,7 @@ func NewRepository(pool *pgxpool.Pool) *Repository {
 // Returns nil, nil when no plan exists for that week.
 func (r *Repository) GetByWeek(ctx context.Context, userID, weekStart string) (*MenuPlan, error) {
 	const q = `
-		SELECT mp.id, mp.week_start,
+		SELECT mp.id, mp.week_start::text,
 		       mi.id, mi.day_of_week, mi.meal_type,
 		       sr.id, sr.title, COALESCE(sr.image_url, ''), sr.nutrition
 		FROM menu_plans mp