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/models/auth.py

@@ -52,7 +52,7 @@ class UpdateScopesResponse(BaseResponse):
 
 
 # All supported application-level scopes
-ALL_SUPPORTED_SCOPES = [
+ALL_SUPPORTED_SCOPES = (
     "notes:read",
     "notes:write",
     "calendar:read",
@@ -73,4 +73,4 @@ ALL_SUPPORTED_SCOPES = [
     "sharing:write",
     "news:read",
     "news:write",
-]
+)