harden: restrict markdown link schemes; parse untrusted XML with defusedxml

Two small defensive-hardening changes:

- web/src/components/Markdown.tsx: render links only for http(s)/mailto
  schemes; other schemes (javascript:, data:, vbscript:) are dropped to
  plain text so a crafted link in rendered content can't execute on click.

- gateway/platforms/wecom_callback.py: parse the untrusted, pre-auth WeCom
  callback request body with defusedxml instead of xml.etree, blocking
  entity-expansion / billion-laughs (and XXE) on the parse path. defusedxml
  is already a dependency (uv.lock); response-building XML in
  wecom_crypto.py is unchanged (it is not parsed from untrusted input).

Verified: dashboard typechecks and builds; defusedxml blocks an
entity-expansion payload while valid WeCom envelopes still parse.

gateway/platforms/wecom_callback.py

@@ -17,7 +17,11 @@ import logging
 import socket as _socket
 import time
 from typing import Any, Dict, List, Optional
-from xml.etree import ElementTree as ET
+# Security: parse untrusted, pre-auth request bodies (WeCom callbacks) with
+# defusedxml to block billion-laughs / entity-expansion (and XXE) DoS. The
+# parsing API (fromstring) is a drop-in for the stdlib calls used below;
+# response-building XML lives in wecom_crypto.py and is not parsed here.
+import defusedxml.ElementTree as ET
 
 try:
     from aiohttp import web

web/src/components/Markdown.tsx

@@ -324,11 +324,24 @@ function InlineContent({
                 <HighlightedText text={node.content} terms={highlightTerms} />
               </em>
             );
-          case "link":
+          case "link": {
+            // Security: only render http(s)/mailto links. Other schemes
+            // (javascript:, data:, vbscript:) are dropped to plain text so a
+            // crafted link in agent/message content can't execute on click.
+            const href = node.href.trim();
+            if (!/^(https?:|mailto:)/i.test(href)) {
+              return (
+                <HighlightedText
+                  key={i}
+                  text={node.text}
+                  terms={highlightTerms}
+                />
+              );
+            }
             return (
               <a
                 key={i}
-                href={node.href}
+                href={href}
                 target="_blank"
                 rel="noreferrer"
                 className="text-primary underline underline-offset-2 decoration-primary/30 hover:decoration-primary/60 transition-colors"
@@ -336,6 +349,7 @@ function InlineContent({
                 {node.text}
               </a>
             );
+          }
           case "br":
             return <br key={i} />;
         }