feat(integrations): add dynamic OpenRouter model loading

- Remove hardcoded OPENROUTER_MODEL_CHOICES from openrouter.py
- Add API endpoint /integrations/openrouter/models/ to fetch models dynamically
- Models loaded from OpenRouter API with free models (':free') at top
- Update OpenRouterIntegration model_name field (remove choices, blank=True)
- Add async buildForm() with dynamic_choices support
- Show asterisks (********) for saved API keys with helpful placeholder

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

myproject/integrations/urls.py

@@ -6,6 +6,7 @@ from .views import (
     get_integration_form_data,
     test_integration_connection,
     RecommerceBatchSyncView,
+    get_openrouter_models,
 )
 
 app_name = 'integrations'
@@ -22,4 +23,7 @@ urlpatterns = [
     
     # Синхронизация
     path("recommerce/sync/", RecommerceBatchSyncView.as_view(), name="recommerce_sync"),
+
+    # OpenRouter модели
+    path("openrouter/models/", get_openrouter_models, name="openrouter_models"),
 ]