fix(signal_v2): await cancelled poll_task + public cache_size

Code review fixes:
- main.py lifespan: await poll_task after cancel() to avoid client
  close racing with mid-fetch task (CRITICAL).
- stock_client: add public cache_size() method; main.py /health uses
  it instead of private _cache attribute (IMPORTANT).

19 tests still pass. Deferred to Phase 7 backlog:
- _ctx singleton test isolation (importlib.reload provides isolation in practice)
- poll_loop interval floor (interval >= 60 by design)
- shutdown logging
- response schema validation

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

signal_v2/main.py

@@ -50,6 +50,10 @@ async def lifespan(app: FastAPI):
             await asyncio.wait_for(_ctx.poll_task, timeout=5.0)
         except asyncio.TimeoutError:
             _ctx.poll_task.cancel()
+            try:
+                await _ctx.poll_task
+            except asyncio.CancelledError:
+                pass
     if _ctx.client is not None:
         await _ctx.client.close()
 
@@ -66,5 +70,5 @@ async def health():
         "status": "online",
         "stock_api_url": settings.stock_api_url,
         "last_poll": state_mod.state.last_updated,
-        "cache_size": len(_ctx.client._cache) if _ctx.client is not None else 0,
+        "cache_size": _ctx.client.cache_size() if _ctx.client is not None else 0,
     }