fix(ocr+bot): image download race, SenseNova context, log path, encoding

Root causes of the screenshot 404 incident:
1. RACE: client uploads image AND sends message concurrently; bot received
   the message before the upload finished writing, so its GET hit a 404
   error page (<100B treated as failure). FIX: _download_image now retries
   3x with 2s backoff.
2. Zhiwei mentioned tesseract/小果 because the failure text never told her
   the pipeline IS SenseNova. FIX: failure messages now name SenseNova
   explicitly and ask for resend.
3. log_xmpp never worked for the bot: sys.path used relative '../..' from
   a symlinked __file__ which resolved to '/' instead of MoFin root. This
   is why the '最近对话' panel never had bot chat data (only cron script
   entries). FIX: absolute path per red line #7. Verified: test message
   now lands in xmpp_messages.jsonl.
4. My PowerShell -replace corrupted the file encoding (UnicodeDecodeError
   crash loop on restart). Restored from git HEAD and re-applied edits with
   the edit tool. Lesson: never use PowerShell string replace on UTF-8
   source files with Chinese content.
5. functional_health: new sense_ocr module (OCR config presence +
   SenseNova API TCP reachability), no token cost.

scripts/test_msg_log.py

@@ -0,0 +1,8 @@
+import json, urllib.request
+payload = json.dumps({"to": "hmo@yoin.fun", "body": "[系统] 消息日志链路自检（请忽略此条）", "type": "chat"}).encode()
+req = urllib.request.Request("http://127.0.0.1:5805/", data=payload, headers={"Content-Type": "application/json"})
+print(urllib.request.urlopen(req, timeout=5).read().decode())
+import time
+time.sleep(2)
+last = open('/home/hmo/MoFin/gateway/logs/xmpp_messages.jsonl').readlines()[-1]
+print('jsonl last:', last[:200])