docs: add thumbnail endpoint to 08_media.md

docs_v1.0/API_WORKSPACE/modules/08_media.md

@@ -85,6 +85,28 @@ curl -s "$API/api/v1/file/$FILE_UUID/trace/1939/representative-face" \
 
 #### Error Responses
 
+---
+
+### `GET /api/v1/file/:file_uuid/trace/:trace_id/thumbnail`
+
+Extract the best face image for a trace as JPEG (320×320). Internally selects the face using the same two-stage algorithm as `representative-face`, then crops via FFmpeg. The result is cacheable for 24 hours.
+
+**Auth**: Required
+**Scope**: file-level
+
+#### Example
+
+```bash
+curl -s "$API/api/v1/file/$FILE_UUID/trace/1939/thumbnail" \
+  -H "X-API-Key: $KEY" -o trace_1939_face.jpg
+```
+
+#### Response
+
+- **200**: `image/jpeg` binary data (320×320 cropped face)
+- **404**: File, trace not found, or no suitable face
+- **500**: FFmpeg or database error
+
 | HTTP | When |
 |------|------|
 | `404` | File, trace not found, or no suitable face |