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.
chore: translate Japanese comments to English for better code readability
Add new endpoint `/mp4/single/:stickerId` to generate MP4 videos combining sound and animation/static images from LINE stickers. The endpoint accepts parameters for device type (ios/android), static/animation mode, and size variant. Implements ffmpeg processing to merge audio and visual components with proper scaling and format conversion. Includes input validation, error handling, and temporary file cleanup.
