fix(auth): address PR #758 round-7 important review

- Coerce refresh_expires_in to int before arithmetic in both callback
  paths so IdPs that serialize the field as a JSON string (e.g. AWS
  Cognito) don't trigger an unhandled TypeError 500.
- Drop the orphaned oauth_session row written by _check_logged_in. The
  canonical Flow 2 row is created by generate_oauth_url_for_flow2 keyed
  by `state`, which is what the unified callback looks up; the
  flow2_<hex> session_id was never matched and just churned the table
  for 10 minutes per call.
- Match delete_cookie attributes (httponly, secure, samesite) to the
  set_cookie call on logout so browsers reliably evict the cookie even
  on implementations that consider security flags during deletion.

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

nextcloud_mcp_server/server/oauth_tools.py

@@ -432,21 +432,12 @@ async def _check_logged_in(ctx: Context, user_id: str) -> str:
         # Store state in session for validation on callback
         storage = await get_shared_storage()
 
-        # Create OAuth session for Flow 2. Identifier is purely random
-        # so audit-log entries / DB rows don't carry the user_id in the
-        # session_id field (PR #758 round-4 review medium 3).
-        session_id = f"flow2_{secrets.token_hex(16)}"
+        # The canonical Flow 2 oauth_session row is written inside
+        # generate_oauth_url_for_flow2 (keyed by `state`, with the PKCE
+        # verifier and nonce); the unified callback looks it up by `state`.
+        # No additional row is needed here.
         redirect_uri = f"{os.getenv('NEXTCLOUD_MCP_SERVER_URL', 'http://localhost:8000')}/oauth/callback"
 
-        await storage.store_oauth_session(
-            session_id=session_id,
-            client_redirect_uri="",  # No client redirect for Flow 2
-            state=state,
-            flow_type="flow2",
-            is_provisioning=True,
-            ttl_seconds=600,  # 10 minute TTL
-        )
-
         # Define scopes for Nextcloud access
         # Note: offline_access is only included when enabled in settings.
         # The actual scope sent to the IdP is determined by