add volcengine OSS support

sentence_api/README.md

@@ -4,6 +4,9 @@ FastAPI service for the Android oral-training SDK. It provides:
 
 - chunked video upload with SHA-256 calculation;
 - files stored under `data/v/` and metadata stored in SQLite;
+- after the first processing pass, videos can be uploaded automatically to
+  Volcano Engine Object Storage (TOS / 火山 OSS) and the catalog/share UI will
+  use that public URL;
 - background sentence transcription through MOSS-Transcribe-Diarize;
 - a browser-based administration page at `/admin`;
 - video catalog and HTTP Range playback;
@@ -33,6 +36,12 @@ http://127.0.0.1:8000/docs
 When `MOSS_TRANSCRIBE_URL` is empty, uploaded videos still receive silence-based
 boundaries, but no reference transcript is produced and assessment is disabled.
 
+When all `VOLCANO_OSS_*` credentials in `.env.example` are configured, the first
+processing pass saves sentence boundaries, uploads the source video, and only
+then marks it ready. `stream_url` and the legacy `/content` endpoint then point
+to the OSS/CDN URL. If the upload fails, the video is marked failed so clients
+do not fall back to server-local playback.
+
 ## Main Endpoints
 
 ```text