Fix image serving: use cache_timeout not max_age (Quart API)

Quart's send_file uses cache_timeout= not max_age=. The TypeError on
every /api/images/<id> request caused a 500, which the browser rendered
as alt text.

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

src/fabledassistant/routes/images.py

@@ -31,5 +31,5 @@ async def serve_image(image_id: int):
     return await send_file(
         file_path,
         mimetype=record.content_type,
-        max_age=86400,
+        cache_timeout=86400,
     )