fix: encode non-ASCII filename in Content-Disposition header

Use RFC 5987 filename*=UTF-8'' percent-encoding to support Chinese and
other Unicode characters in DOCX download filenames.

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

app/api/v1/endpoints/convert.py

@@ -1,5 +1,7 @@
 """Format conversion endpoints."""
 
+from urllib.parse import quote
+
 from fastapi import APIRouter, Depends, HTTPException
 from fastapi.responses import Response
 
@@ -34,10 +36,11 @@ async def convert_markdown_to_docx(
             request.filename,
             len(docx_bytes),
         )
+        encoded_name = quote(f"{request.filename}.docx")
         return Response(
             content=docx_bytes,
             media_type="application/vnd.openxmlformats-officedocument.wordprocessingml.document",
-            headers={"Content-Disposition": f'attachment; filename="{request.filename}.docx"'},
+            headers={"Content-Disposition": f"attachment; filename*=UTF-8''{encoded_name}"},
         )
     except Exception as e:
         logger.exception("DOCX conversion failed, filename=%s: %s", request.filename, e)