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/models/auth.py

@@ -51,26 +51,28 @@ class UpdateScopesResponse(BaseResponse):
     new_scopes: list[str] | None = Field(None, description="Updated scope set")
 
 
-# All supported application-level scopes
-ALL_SUPPORTED_SCOPES = (
-    "notes:read",
-    "notes:write",
-    "calendar:read",
-    "calendar:write",
-    "todo:read",
-    "todo:write",
-    "contacts:read",
-    "contacts:write",
-    "files:read",
-    "files:write",
-    "tables:read",
-    "tables:write",
-    "deck:read",
-    "deck:write",
-    "cookbook:read",
-    "cookbook:write",
-    "sharing:read",
-    "sharing:write",
-    "news:read",
-    "news:write",
+# All supported application-level scopes (frozenset for O(1) membership tests)
+ALL_SUPPORTED_SCOPES: frozenset[str] = frozenset(
+    {
+        "notes:read",
+        "notes:write",
+        "calendar:read",
+        "calendar:write",
+        "todo:read",
+        "todo:write",
+        "contacts:read",
+        "contacts:write",
+        "files:read",
+        "files:write",
+        "tables:read",
+        "tables:write",
+        "deck:read",
+        "deck:write",
+        "cookbook:read",
+        "cookbook:write",
+        "sharing:read",
+        "sharing:write",
+        "news:read",
+        "news:write",
+    }
 )