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/token_utils.py

@@ -0,0 +1,85 @@
+"""Token utility functions for extracting user identity from MCP access tokens.
+
+Extracted from server/oauth_tools.py to break circular import dependencies
+between server/ and auth/ layers.
+"""
+
+import logging
+import os
+
+import jwt
+from mcp.server.auth.middleware.auth_context import get_access_token
+from mcp.server.auth.provider import AccessToken
+from mcp.server.fastmcp import Context
+
+from nextcloud_mcp_server.auth.userinfo_routes import _query_idp_userinfo
+
+from ..http import nextcloud_httpx_client
+
+logger = logging.getLogger(__name__)
+
+
+async def extract_user_id_from_token(ctx: Context) -> str:
+    """Extract user_id from the MCP access token (Flow 1).
+
+    Handles both JWT and opaque tokens:
+    - JWT: Decode and extract 'sub' claim
+    - Opaque: Call userinfo endpoint to get 'sub'
+
+    Args:
+        ctx: MCP context with access token
+
+    Returns:
+        user_id extracted from token, or "default_user" as fallback
+    """
+    # Use MCP SDK's get_access_token() which uses contextvars
+    access_token: AccessToken | None = get_access_token()
+
+    if not access_token or not access_token.token:
+        logger.warning("  ✗ No access token found via get_access_token()")
+        return "default_user"
+
+    token = access_token.token
+    is_jwt = "." in token and token.count(".") >= 2
+    logger.info(f"  Token type: {'JWT' if is_jwt else 'Opaque'}")
+
+    # Try JWT decode first
+    if is_jwt:
+        try:
+            payload = jwt.decode(token, options={"verify_signature": False})
+            user_id = payload.get("sub", "unknown")
+            logger.info(f"  ✓ JWT decode successful: user_id={user_id}")
+            return user_id
+        except Exception as e:
+            logger.error(f"  ✗ JWT decode failed: {type(e).__name__}: {e}")
+
+    # Opaque token - call userinfo endpoint
+    logger.info("  Opaque token detected, calling userinfo endpoint...")
+    try:
+        # Get userinfo endpoint from OIDC discovery
+        oidc_discovery_uri = os.getenv(
+            "OIDC_DISCOVERY_URI",
+            "http://localhost:8080/.well-known/openid-configuration",
+        )
+        async with nextcloud_httpx_client() as http_client:
+            discovery_response = await http_client.get(oidc_discovery_uri)
+            discovery_response.raise_for_status()
+            discovery = discovery_response.json()
+            userinfo_endpoint = discovery.get("userinfo_endpoint")
+
+        if userinfo_endpoint:
+            userinfo = await _query_idp_userinfo(token, userinfo_endpoint)
+            if userinfo:
+                user_id = userinfo.get("sub", "unknown")
+                logger.info(f"  ✓ Userinfo query successful: user_id={user_id}")
+                return user_id
+            else:
+                logger.error("  ✗ Userinfo query failed")
+        else:
+            logger.error("  ✗ No userinfo_endpoint available")
+    except Exception as e:
+        logger.error(f"  ✗ Userinfo query failed: {type(e).__name__}: {e}")
+
+    # Fallback
+    logger.warning("  Using fallback user_id: default_user")
+    return "default_user"