add chinese subtitles

sentence_api/README.md

@@ -100,6 +100,11 @@ The endpoint requires a signed-in user and accepts only the sentence and course
 context. `AI_API_URL` and `AI_API_KEY` remain on the server and are never sent
 to the mobile clients.
 
+`POST /api/v1/videos/{sha256}/subtitle-translation` translates the full English
+transcript in one AI request, then stores one Chinese caption per sentence.
+Later requests reuse the cached Chinese captions until the English sentences
+change.
+
 ## Assessment
 
 ```bash