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/tests/test_worker.py

@@ -94,3 +94,25 @@ async def test_poll_once_returns_false_on_timeout(monkeypatch):
     assert handled is False
     fake_queue.ack.assert_not_awaited()
     fake_queue.fail.assert_not_awaited()
+
+
+# ----- heartbeat stats 카운터 -----
+
+class _OneJobQueue:
+    def __init__(self): self.acked = False
+    async def dequeue(self, timeout=5):
+        if self.acked: return None
+        return ({"job_type": "sora_generation", "task_id": "t1", "params": {}}, b"raw")
+    async def ack(self, raw): self.acked = True
+    async def fail(self, raw, payload): pass
+
+
+@pytest.mark.asyncio
+async def test_poll_once_increments_jobs_done(monkeypatch):
+    worker.stats.jobs_done = 0
+    monkeypatch.setattr(worker, "run_sora_generation", lambda task_id, params: None)
+    handled = await worker.poll_once(_OneJobQueue())
+    assert handled is True
+    assert worker.stats.jobs_done == 1
+    assert worker.stats.busy is False
+    assert worker.stats.last_job_at is not None