add sentences borders enable in pc player

sentence_api/README.md

@@ -52,6 +52,7 @@ GET    /api/v1/videos
 GET    /api/v1/videos/{sha256}
 GET    /api/v1/videos/{sha256}/content
 GET    /api/v1/videos/{sha256}/sentence-boundaries
+POST   /api/v1/audio/sentence-boundaries
 POST   /api/v1/sentences/explain
 POST   /api/v1/videos/{sha256}/sentences/{index}/assessments
 
@@ -67,6 +68,11 @@ DELETE /api/v1/admin/videos/{sha256}
 Admin endpoints use `X-Admin-Key` when `ADMIN_API_KEY` is configured. Assessment
 requests use `X-Client-Key` when `CLIENT_API_KEY` is configured.
 
+`POST /api/v1/audio/sentence-boundaries` accepts an audio file from an
+authenticated client, generates boundaries in memory/temp storage only, and
+returns a sentence document. It does not register or archive the uploaded
+audio.
+
 ## AI Sentence Explanation
 
 Configure an OpenAI-compatible chat-completions endpoint in `sentence_api/.env`.