fix: increase connectTimeout and replace expires_at stored column refs

- api_client.dart: connectTimeout 10s → 60s — Flutter was cancelling
  requests to /recommendations after 10s while the server waited for
  OpenAI, causing 'context canceled' on the server side
- product/repository.go ListForPrompt: expires_at is computed via
  (added_at + storage_days * INTERVAL '1 day'), not a stored column;
  wrap in CTE to reference it by alias in SELECT/ORDER BY
- home/handler.go getExpiringSoon: same fix — use CTE to compute
  expires_at before filtering/ordering by it

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

backend/internal/home/handler.go

@@ -150,12 +150,16 @@ func defaultPlan() []MealPlan {
 // getExpiringSoon returns products expiring within the next 3 days.
 func (h *Handler) getExpiringSoon(ctx context.Context, userID string) []ExpiringSoon {
 	rows, err := h.pool.Query(ctx, `
+		WITH p AS (
+			SELECT name, quantity, unit,
+			       (added_at + storage_days * INTERVAL '1 day') AS expires_at
+			FROM products
+			WHERE user_id = $1
+		)
 		SELECT name, quantity, unit,
 		       GREATEST(0, EXTRACT(EPOCH FROM (expires_at - now())) / 86400)::int
-		FROM products
-		WHERE user_id = $1
-		  AND expires_at IS NOT NULL
-		  AND expires_at > now()
+		FROM p
+		WHERE expires_at > now()
 		  AND expires_at <= now() + INTERVAL '3 days'
 		ORDER BY expires_at
 		LIMIT 5`,