add explain module

sentence_api/README.md

@@ -12,6 +12,7 @@ FastAPI service for the Android oral-training SDK. It provides:
 - video catalog and HTTP Range playback;
 - sentence-boundary lookup compatible with the existing Android SDK;
 - student recording upload and `asr-fluency-v1` assessment;
+- AI sentence explanation for signed-in mobile users;
 - the legacy JSON boundary index as a read-only fallback.
 
 ## Local Run
@@ -50,6 +51,7 @@ 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/sentences/explain
 POST   /api/v1/videos/{sha256}/sentences/{index}/assessments
 
 POST   /api/v1/admin/videos
@@ -64,6 +66,21 @@ 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.
 
+## AI Sentence Explanation
+
+Configure an OpenAI-compatible chat-completions endpoint in `sentence_api/.env`:
+
+```dotenv
+AI_API_URL=https://api.example.com/v1/chat/completions
+AI_API_KEY=replace-with-your-ai-api-key
+AI_MODEL=gpt-4o-mini
+AI_TIMEOUT_SECONDS=45
+```
+
+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.
+
 ## Assessment
 
 ```bash