fix: address remaining PR #589 review findings

- Consolidate MCP session + login flow cleanup into _mcp_session_with_login_flow() helper,
  replacing 4 duplicated AsyncExitStack sites in app.py
- Fix get_shared_storage() race condition by using module-level anyio.Lock() init
  (reverts regression from ba59763)
- Collapse cosmetic if/else branching in scope_authorization.py
- Consolidate dual password storage paths into single store_app_password_with_scopes() call
- Mark unused request param as _ in list_supported_scopes
- Make ALL_SUPPORTED_SCOPES an immutable tuple; use list() instead of .copy()
- Add hasattr(ctx, "elicit") guard in elicitation.py, narrow except to NotImplementedError
- Add YAML comment explaining --oauth flag for mcp-login-flow service

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

nextcloud_mcp_server/api/access.py

@@ -154,7 +154,7 @@ async def update_user_scopes(request: Request) -> JSONResponse:
         )
 
 
-async def list_supported_scopes(request: Request) -> JSONResponse:
+async def list_supported_scopes(_: Request) -> JSONResponse:
     """GET /api/v1/scopes - List all supported application-level scopes."""
     return JSONResponse(
         {