feat(api): add sticker video download endpoint documentation

Added documentation for the new sticker video (MP4) download endpoint including path parameters, query parameters, and supported device types. This endpoint allows downloading video files that combine sticker images and sounds.

API.md

@@ -168,6 +168,26 @@ Download the main sound file for a sticker product.
 - `device_type`: string (optional, default: 'ios')
   - Values: 'ios', 'android', 'pc'
 
+### Sticker video (MP4)
+
+`GET /api/download/sticker/mp4/single/:stickerId`
+
+Download a video file combining sticker image and sound.
+
+#### Path Parameters
+
+- `stickerId`: number (integer)
+  - Individual sticker ID
+
+#### Query Parameters
+
+- `device_type`: string (optional, default: 'ios')
+  - Values: 'ios', 'android'
+- `is_static`: boolean (optional, default: false)
+  - If true, downloads static version
+- `size`: number (optional, default: 2)
+  - Values: 1 (normal), 2 (high resolution)
+
 ---
 
 ## Download Emojis