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/stock_client.py

@@ -34,6 +34,10 @@ class StockClient:
     async def close(self) -> None:
         await self._client.aclose()
 
+    def cache_size(self) -> int:
+        """Number of cached endpoint responses (public surface for /health)."""
+        return len(self._cache)
+
     async def get_portfolio(self) -> dict:
         return await self._cached_request(
             "portfolio", "GET", "/api/webai/portfolio"