fix(image-render): F6 ReliableQueue 적용 (F6 part 5)

- worker.py: poll_once + ReliableQueue + startup recovery
- 3 provider (gpt_image/nano_banana/flux) dispatch table 보존
- Dockerfile: build context=services/, _shared 포함, PYTHONPATH=/app
- docker-compose.yml: image-render build context 갱신

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

services/docker-compose.yml

@@ -101,7 +101,8 @@ services:
 
   image-render:
     build:
-      context: ./image-render
+      context: .
+      dockerfile: image-render/Dockerfile
     container_name: image-render
     restart: unless-stopped
     ports: