fix: guard AI tidy verdict against non-string LLM output (#1486)

The AI document-tidy endpoint parses verdicts from LLM JSON output
and calls .lower().strip() directly. If the model returns null or a
non-string element, this crashes with AttributeError. Coerce to str
so malformed output is treated as 'keep' instead of crashing.

routes/document_routes.py

@@ -901,7 +901,7 @@ def setup_document_routes(session_manager, upload_handler=None) -> APIRouter:
             for i, doc in enumerate(batch):
                 if i >= len(verdicts):
                     break
-                verdict = verdicts[i].lower().strip()
+                verdict = str(verdicts[i] or "").lower().strip()
                 if verdict == "junk":
                     doc.tidy_verdict = "junk"
                     db.delete(doc)