Add refresh token functionality and improve login response handling

- Introduced RefreshTokenView to allow users to refresh their access tokens using a valid refresh token.
- Enhanced LoginView to ensure correct user/account loading and improved error handling during user serialization.
- Updated API response structure to include access and refresh token expiration times.
- Adjusted frontend API handling to support both new and legacy token response formats.

backend/igny8_core/modules/billing/views.py

@@ -54,8 +54,8 @@ class CreditBalanceViewSet(viewsets.ViewSet):
                 request=request
             )
         
-        # Get plan credits per month
-        plan_credits_per_month = account.plan.credits_per_month if account.plan else 0
+        # Get plan credits per month (use get_effective_credits_per_month for Phase 0 compatibility)
+        plan_credits_per_month = account.plan.get_effective_credits_per_month() if account.plan else 0
         
         # Calculate credits used this month
         now = timezone.now()