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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>