feat(render-workers): 4 render 워커 heartbeat 배선 + poll_once 카운터

- services/_shared/heartbeat.py (A1) WorkerStats/utc_now_iso/heartbeat_loop 소비
- image-render / video-render / music-render / insta-render 각 worker.py:
  stats = WorkerStats() 모듈 레벨 추가, poll_once에서 dispatch 전 busy=True,
  ack 후 jobs_done+1 / fail 후 jobs_failed+1 + last_job_at + busy=False
- 각 main.py: lifespan에 aioredis(decode_responses=False) + heartbeat_loop 태스크 spawn,
  종료 시 cancel + aclose
- 각 tests/test_worker.py: test_poll_once_increments_jobs_done 추가
  (image:flux / video:sora / music:suno / insta:_process_one mock)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019LV86jBozkNhSFXJA412fq

services/insta-render/worker.py

@@ -14,9 +14,11 @@ import redis.asyncio as aioredis
 
 from card_renderer import render_slate
 from _shared.reliable_queue import ReliableQueue
+from _shared.heartbeat import WorkerStats, utc_now_iso
 
 logger = logging.getLogger(__name__)
 
+stats = WorkerStats()
 
 REDIS_URL = os.getenv("REDIS_URL", "redis://192.168.45.54:6379")
 NAS_BASE_URL = os.getenv("NAS_BASE_URL", "http://192.168.45.54:18700")
@@ -89,12 +91,19 @@ async def poll_once(queue: ReliableQueue, client: httpx.AsyncClient) -> bool:
     if result is None:
         return False
     payload, raw = result
+    stats.busy = True
     try:
         await _process_one(client, payload)
     except Exception:
         await queue.fail(raw, payload)
+        stats.jobs_failed += 1
+        stats.last_job_at = utc_now_iso()
+        stats.busy = False
         return True
     await queue.ack(raw)
+    stats.jobs_done += 1
+    stats.last_job_at = utc_now_iso()
+    stats.busy = False
     return True