fix: address review feedback — security, caching, CI 429 retry

- Add 429 retry with exponential backoff to register_client() (fixes CI
  oauth matrix failures from parallel DCR requests)
- Make client_id, redirect_uri, and PKCE mandatory at token endpoint
- Add null-checks for discovery_url and OAuth credentials in proxy flows
- Add OIDC discovery document caching with 5-min TTL
- Add per-IP rate limiting on /oauth/register DCR proxy
- Discover DCR endpoint from OIDC discovery instead of hardcoding
- Extract extract_user_id_from_token to auth/token_utils.py (breaks
  circular imports between server/ and auth/ layers)
- Add TTL scope cache in scope_authorization.py (avoids DB hit per tool)
- Add defense-in-depth scope validation in storage layer
- Broaden elicitation exception handling with graceful fallback
- Add idempotentHint to nc_auth_check_status, return "pending" status
  after accepted elicitation, add polling interval to description
- Change ALL_SUPPORTED_SCOPES from tuple to frozenset for O(1) lookups
- Replace Optional[str] with str | None throughout config.py
- Use default_factory for ProxyCodeEntry/ASProxySession dataclasses
- Add proxy code/session cleanup to background loop
- Fix OIDC verification CI step to only run for oauth/login-flow modes
- Add unit tests for access.py REST endpoints (10 tests)

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

nextcloud_mcp_server/auth/elicitation.py

@@ -76,9 +76,13 @@ async def present_login_url(
             logger.info("User cancelled login flow")
             return "cancelled"
 
-    except NotImplementedError as e:
+    except NotImplementedError:
         # Elicitation not supported by this client/SDK - fall back to message
-        logger.debug(
-            f"Elicitation not available ({type(e).__name__}: {e}), returning URL in message"
+        logger.debug("Elicitation not available, returning URL in message")
+        return "message_only"
+    except Exception as e:
+        logger.warning(
+            f"Elicitation failed unexpectedly ({type(e).__name__}: {e}), "
+            "falling back to message"
         )
         return "message_only"