feat(health): recent XMPP conversation log panel + stale error fix

User requirement: health tab should show recent XMPP conversations.
- bot hooks log_xmpp on inbound (on_msg) and outbound (_deliver_loop)
  so real chats land in xmpp_messages.jsonl (was: only cron/scanner)
- index.html health tab: new '最近对话' panel (last 10 msgs, dir arrow,
  preview, status, time); refreshHealth updates it incrementally
- last_error now shows age and resolved state: once a successful
  outbound happens after an error, it's shown gray as '已恢复'
  instead of alarming red forever; unresolved errors still red
- health() status no longer degraded by errors that were later
  resolved by successful outbound

scripts/verify_health_fix.py

@@ -0,0 +1,14 @@
+import urllib.request, json
+d = json.loads(urllib.request.urlopen('http://127.0.0.1:8899/api/xmpp/health', timeout=120).read())
+ba = d.get('bot_activity', {})
+print('status:', d.get('status'))
+print('last_error:', (ba.get('last_error') or '')[:80])
+print('last_error_age_sec:', ba.get('last_error_age_sec'))
+print('last_error_resolved:', ba.get('last_error_resolved'))
+print('last_outbound_age_sec:', ba.get('last_outbound_age_sec'))
+print('---')
+m = json.loads(urllib.request.urlopen('http://127.0.0.1:8899/api/xmpp/messages', timeout=30).read())
+msgs = m.get('messages', [])
+print('messages count:', len(msgs))
+for x in msgs[:5]:
+    print(' ', x.get('timestamp'), x.get('direction'), (x.get('body_preview') or '')[:50])