refactor(auth): Decouple BasicAuth and OAuth authentication strategies

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

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

pyproject.toml

@@ -64,7 +64,7 @@ Changelog = "https://github.com/cbcoutinho/nextcloud-mcp-server/blob/master/CHAN
 
 [tool.pytest.ini_options]
 anyio_mode = "auto"
-addopts = "-p no:asyncio -x"  # Disable pytest-asyncio plugin, use only anyio
+addopts = "-p no:asyncio"  # Disable pytest-asyncio plugin, use only anyio
 log_cli = 1
 log_cli_level = "ERROR"
 log_level = "ERROR"