feat(step-3.6): cloud provider integrations (Gmail, Outlook, Teams)

- Add app/integrations/__init__.py: Fernet token encryption helpers,
  EmailMessage/ChatMessage dataclasses, get_provider() factory
- Add app/integrations/gmail.py: GmailClient with async fetch_messages(),
  token refresh, configurable label/sender/date filters
- Add app/integrations/ms_graph.py: MSGraphClient with fetch_emails()
  (Outlook) and fetch_messages() (Teams), MSAL token refresh, OData filters
- Update app/core/agent_runner.py: replace run_cloud_agent() stub with
  full 8-step implementation; extend _finalize_run() for cloud config type
- Update app/config/settings.py: add OAuth + Fernet encryption settings
- Update requirements.txt: google-api-python-client, google-auth-*,
  msal, cryptography
- Add tests/test_integrations.py: 47 tests covering all integration code
- Update tests/test_agent_runner.py: replace stub test with 7 real tests

All 76 new/updated tests pass.

requirements.txt

@@ -25,4 +25,10 @@ moto[s3]>=5.0.0
 pinecone>=5.0.0
 qdrant-client>=1.7.0
 croniter>=3.0.0
+google-api-python-client>=2.130.0
+google-auth>=2.29.0
+google-auth-oauthlib>=1.2.0
+google-auth-httplib2>=0.2.0
+msal>=1.28.0
+cryptography>=42.0.0
 ruff>=0.8.0