Completely separates multi-user BasicAuth mode from OAuth mode with no
fallback between them. These are now mutually exclusive authentication
strategies based on deployment configuration.
Changes:
- Create separate functions: get_user_client_basic_auth() and
get_user_client_oauth() with clear separation of concerns
- Update get_user_client() to dispatch based on use_basic_auth parameter
- Pass use_basic_auth through all background sync tasks
- Update app.py to determine auth mode at startup
- Rewrite integration tests to verify no OAuth fallback in BasicAuth mode
- Fix test assertions for response field names and duplicate title handling
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
