fix: retry words request on 400/404 + add backend diagnostic logging

Frontend: retry /words POST once after 2s delay if it gets 400/404,
which happens when navigating via wizard after container restart
(session cache not yet warm).

Backend: log when session needs DB reload and when dewarped_bgr is missing.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

klausur-service/backend/ocr_pipeline_api.py

@@ -1204,11 +1204,14 @@ async def detect_words(
         stream: false (default) for JSON response, true for SSE streaming
     """
     if session_id not in _cache:
+        logger.info("detect_words: session %s not in cache, loading from DB", session_id)
         await _load_session_to_cache(session_id)
     cached = _get_cached(session_id)
 
     dewarped_bgr = cached.get("dewarped_bgr")
     if dewarped_bgr is None:
+        logger.warning("detect_words: dewarped_bgr is None for session %s (cache keys: %s)",
+                       session_id, [k for k in cached.keys() if k.endswith('_bgr')])
         raise HTTPException(status_code=400, detail="Dewarp must be completed before word detection")
 
     session = await get_session_db(session_id)