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/tests/test_worker.py

@@ -230,3 +230,27 @@ def test_make_queue_redis_socket_timeout_exceeds_block():
     c = worker.make_queue_redis()
     st = c.connection_pool.connection_kwargs.get("socket_timeout")
     assert st is not None and st > 5   # blmove 블록(5s)보다 커야 안정
+
+
+# ----- heartbeat stats 카운터 -----
+
+class _OneJobQueueInsta:
+    def __init__(self): self.acked = False
+    async def dequeue(self, timeout=5):
+        if self.acked: return None
+        return ({"task_id": "t1", "params": {"slate_id": 1, "theme": "default"}}, 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
+    async def fake_process(client, payload): pass
+    monkeypatch.setattr(worker, "_process_one", fake_process)
+    async with httpx.AsyncClient() as client:
+        handled = await worker.poll_once(_OneJobQueueInsta(), client)
+    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