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_bot_import.py

@@ -0,0 +1,14 @@
+import sys, os
+sys.path.insert(0, '/home/hmo/MoFin/deploy/bot')
+sys.path.insert(0, '/home/hmo/MoFin/deploy/bot/../..')
+try:
+    from xmpp_logger import log_xmpp
+    print('import OK from bot path')
+    import xmpp_logger
+    print('xmpp_logger file:', xmpp_logger.__file__)
+except Exception as e:
+    print('import FAIL:', e)
+
+# also check the file exists
+p = '/home/hmo/MoFin/xmpp_logger.py'
+print('exists:', os.path.exists(p), 'size:', os.path.getsize(p) if os.path.exists(p) else 0)