AssetFile.preview_url was stored blank on upload, so all thumbnails fell back to placeholders.
Make preview_url a SerializerMethodField that signs a TOS GET URL from object_key on read
(falls back to stored value, or "" when TOS unconfigured / no key). Verified: presigned URL
for an existing object returns HTTP 200 image/png.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
