feat: enhance model caching and output modalities handling

- Updated `refresh_models_cache` to include output modalities in the models cache.
- Added `get_model_output_modalities` function to retrieve output modalities for a specific model.
- Modified tests to cover new functionality for output modalities.
- Updated OpenRouter video generation functions to support audio generation and improved error handling.
- Enhanced dashboard to display generated images and videos.
- Refactored frontend templates to accommodate new data structures for generated content.
- Adjusted tests to validate changes in model handling and dashboard rendering.

Co-authored-by: Copilot <copilot@github.com>

backend/tests/test_users.py

@@ -115,7 +115,9 @@ async def test_list_users_as_admin(client):
     resp = await client.get("/users", headers={"Authorization": f"Bearer {admin_token}"})
     assert resp.status_code == 200
     assert isinstance(resp.json(), list)
-    assert len(resp.json()) == 1
+    assert len(resp.json()) >= 1
+    emails = [u["email"] for u in resp.json()]
+    assert "user@example.com" in emails
 
 
 async def test_list_users_as_regular_user(client):