Fix vision attachment timeout and stale cache

Increase local vision model timeout and avoid caching transient VL failure placeholders.\n\nCloses #202.

src/document_processor.py

@@ -230,7 +230,7 @@ def analyze_image_with_vl_result(image_path: str) -> dict:
         last_err = None
         for i, (_url, _model, _headers) in enumerate([c for c in _vl_candidates if c and c[0] and c[1]]):
             try:
-                description = llm_call(_url, _model, vl_messages, headers=_headers, timeout=30)
+                description = llm_call(_url, _model, vl_messages, headers=_headers, timeout=120)
                 logger.info("VL analysis complete with model %s", _model)
                 return {"text": description, "model": _model}
             except Exception as e: