fix(video-render): F6 ReliableQueue 적용 (F6 part 4)

- worker.py: poll_once + ReliableQueue + startup recovery
- 4 provider (sora/veo/kling/seedance) dispatch table 보존
- Dockerfile: build context=services/, _shared 포함, PYTHONPATH=/app
- docker-compose.yml: video-render build context 갱신

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

services/video-render/conftest.py

@@ -0,0 +1,5 @@
+"""Make services/ root importable so `from _shared.reliable_queue import ...` works during tests."""
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))