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/video-render/worker.py

@@ -19,6 +19,7 @@ from providers.veo import run_veo_generation
 from providers.kling import run_kling_generation
 from providers.seedance import run_seedance_generation
 from _shared.reliable_queue import ReliableQueue
+from _shared.heartbeat import WorkerStats, utc_now_iso
 
 logger = logging.getLogger(__name__)
 
@@ -26,6 +27,8 @@ REDIS_URL = os.getenv("REDIS_URL", "redis://192.168.45.54:6379")
 QUEUE_KEY = "queue:video-render"
 PAUSED_KEY = "queue:paused"
 
+stats = WorkerStats()
+
 # string names so `unittest.mock.patch` on `worker.<name>` is correctly intercepted
 _DISPATCH_TABLE = {
     "sora_generation":     "run_sora_generation",
@@ -60,14 +63,21 @@ async def poll_once(queue: ReliableQueue) -> bool:
     if result is None:
         return False
     payload, raw = result
+    stats.busy = True
     try:
         await asyncio.to_thread(_dispatch, payload)
     except Exception:
         logger.exception("dispatch unhandled exception task_id=%s",
                          payload.get("task_id"))
         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