fix(auth): address PR #758 round-6 medium/low review

Five findings from the latest review on #758 (2 medium, 3 nit):

Medium:
- browser_oauth_routes.oauth_login_callback + oauth_routes.oauth_callback_nextcloud:
  fail closed with 400 when the oauth_session row is unknown/expired. Previously
  both callbacks fell through with code_verifier="" and expected_nonce=None,
  silently bypassing the PKCE + nonce protections introduced in earlier rounds.
  Symmetric unit tests pin both contracts.
- token_utils.verify_id_token: use secrets.compare_digest for the nonce check
  instead of short-circuit !=. Mirrors the sibling PKCE verifier comparison;
  closes the last secret-equality timing-side-channel surface in the auth path.

Nit:
- Tighten the comment at all 4 mcp_authorization_code/code_verifier store +
  retrieve sites so a future refactor sees the field reuse immediately
  (renaming the column requires a schema migration).
- _should_use_secure_cookies: explicit string normalisation instead of
  bool(settings.cookie_secure). Dynaconf normally coerces but tests / direct
  settings.set calls can leave the raw string in place — bool("false") is True.
  New parametrized unit tests cover the coercion matrix + http/https fallback.
- oauth_routes.py:591 f-string log converted to lazy %s formatting (folded into
  the Flow 2 callback rewrite).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/auth/token_utils.py

@@ -5,6 +5,7 @@ between server/ and auth/ layers.
 """
 
 import logging
+import secrets
 import time
 from typing import Any
 
@@ -232,7 +233,13 @@ async def verify_id_token(
             f"Unexpected error verifying ID token: {e}"
         ) from e
 
-    if expected_nonce is not None and payload.get("nonce") != expected_nonce:
+    # Constant-time comparison mirrors the PKCE verifier check
+    # (oauth_routes.py:1029) — short-circuit `!=` is avoided in
+    # security-sensitive equality even when the secret is server-generated
+    # (round-6 review).
+    if expected_nonce is not None and not secrets.compare_digest(
+        payload.get("nonce", "") or "", expected_nonce
+    ):
         raise IdTokenVerificationError("ID token nonce does not match request nonce")
 
     return payload