fix(auth): fail closed on missing sub claim, delete Flow 2 callback session

Addresses the two remaining 🟡 findings from the PR #758 follow-up review:

  1. extract_user_id_from_token previously fell back to "default_user" when
     the verified access token had no sub claim. In a multi-tenant deployment
     a malformed IdP token could have bucketed every request under a single
     sentinel user, risking cross-tenant data exposure. The function now
     raises McpError on that branch; the BasicAuth no-token sentinel path is
     preserved.

  2. oauth_callback_nextcloud (Flow 2) read the PKCE code_verifier from
     oauth_sessions but never deleted the row, leaving the verifier valid for
     the full 10-minute TTL. The row is now deleted eagerly inside the same
     branch, mirroring oauth_login_callback in browser_oauth_routes.

Also wires TOKEN_ENCRYPTION_KEY through the docker-compose step in the CI
test workflow so the integration matrix can boot — every job had been
failing fast on the ${TOKEN_ENCRYPTION_KEY:?...} interpolation guard added
in PR #758 finding 5.

Tests pin both fixes (test_token_utils_user_id.py,
test_oauth_callback_session_cleanup.py).

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

nextcloud_mcp_server/auth/oauth_routes.py

@@ -597,6 +597,11 @@ async def oauth_callback_nextcloud(request: Request):
         logger.info(
             f"Retrieved code_verifier for Flow 2 callback (state={state[:16]}...)"
         )
+        # One-time-use session: delete eagerly so the stored code_verifier
+        # can't be replayed for the remainder of the oauth_sessions TTL.
+        # Mirrors browser_oauth_routes.oauth_login_callback (PR #758
+        # follow-up review).
+        await storage.delete_oauth_session(state)
 
     # Exchange code for tokens
     mcp_server_client_id = os.getenv(