fix: treat 404 on Alpaca positions endpoint as empty result

Alpaca returns 404 when you don't hold a position in a ticker.
The ingestion worker was logging this as an error and incrementing
the failure count. Now returns an empty items list instead, since
'no position' is a valid state, not an error.

services/adapters/broker_adapter.py

@@ -364,6 +364,19 @@ class AlpacaBrokerAdapter(BrokerDataAdapter):
                 )
             except httpx.HTTPStatusError as e:
                 elapsed_ms = (time.monotonic() - t0) * 1000
+                # 404 on positions endpoint means no position held — not an error
+                if e.response is not None and e.response.status_code == 404 and endpoint == "positions":
+                    return AdapterResult(
+                        source_type="broker",
+                        ticker=ticker,
+                        items=[],
+                        raw_payload=b"[]",
+                        content_hash=hashlib.sha256(b"[]").hexdigest(),
+                        fetched_at=datetime.now(timezone.utc),
+                        http_status=404,
+                        response_time_ms=round(elapsed_ms, 1),
+                        metadata={"provider": "alpaca", "mode": self._mode.value, "endpoint": endpoint},
+                    )
                 logger.error("Alpaca HTTP error for %s: %s", ticker, e)
                 return self._error_result(
                     ticker, str(e), elapsed_ms,