fix(music-render): F6 ReliableQueue 적용 (F6 part 3)

- worker.py: poll_once 신설, BLPOP → ReliableQueue.dequeue/ack/fail + startup recovery
- 12 job_type dispatch table 보존 (기존 13 tests 그대로 PASS)
- Dockerfile: build context=services/, _shared 포함, PYTHONPATH=/app
- docker-compose.yml: music-render build context 갱신

dispatch 자체 unhandled exception 발생 시 fail(raw, payload)로 retry/dead-letter.
provider 함수가 webhook("failed")를 잡고 있는 정상 케이스는 ack (멱등 webhook).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

services/docker-compose.yml

@@ -27,7 +27,8 @@ services:
 
   music-render:
     build:
-      context: ./music-render
+      context: .
+      dockerfile: music-render/Dockerfile
     container_name: music-render
     restart: unless-stopped
     ports: