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>

client/lib/core/api/api_client.dart

@@ -8,7 +8,7 @@ class ApiClient {
   ApiClient({required String baseUrl, required SecureStorageService storage}) {
     _dio = Dio(BaseOptions(
       baseUrl: baseUrl,
-      connectTimeout: const Duration(seconds: 10),
+      connectTimeout: const Duration(seconds: 60),
       receiveTimeout: const Duration(seconds: 120),
       headers: {'Content-Type': 'application/json'},
     ));