test(services/insta-render): worker unit tests (3 cases)

- _post_update payload·헤더 검증
- _process_one 정상 흐름 (processing + succeeded)
- _process_one 예외 시 failed webhook

Plan-B-Insta Phase 2 mature. Phase 3 cutover 준비 완료.

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

services/insta-render/pytest.ini

@@ -0,0 +1,2 @@
+[pytest]
+asyncio_mode = auto