feat(signal_v2): pull_worker + FastAPI app + 2 integration tests

poll_loop: asyncio.gather parallel fetch of 3 endpoints (portfolio,
news_sentiment, screener_preview) + state update. main.py: FastAPI
lifespan creates StockClient/SignalDedup/shutdown.Event then spawns
poll_loop as background task. GET /health reports status, last poll
times, cache size.

Signal V2 test suite: 19/19 PASS.

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

signal_v2/tests/test_main.py

@@ -0,0 +1,36 @@
+"""Tests for FastAPI main app."""
+import logging
+
+import pytest
+from fastapi.testclient import TestClient
+
+
+def test_health_endpoint_returns_status_online(monkeypatch):
+    monkeypatch.setenv("STOCK_API_URL", "https://test.stock.local")
+    monkeypatch.setenv("WEBAI_API_KEY", "test-secret")
+    # Reload modules so they pick up the new env
+    import importlib
+    from signal_v2 import config as cfg
+    importlib.reload(cfg)
+    from signal_v2 import main as main_mod
+    importlib.reload(main_mod)
+    with TestClient(main_mod.app) as client:
+        r = client.get("/health")
+        assert r.status_code == 200
+        body = r.json()
+        assert body["status"] == "online"
+        assert body["stock_api_url"] == "https://test.stock.local"
+
+
+def test_startup_warns_if_webai_api_key_missing(monkeypatch, caplog):
+    monkeypatch.delenv("WEBAI_API_KEY", raising=False)
+    monkeypatch.setenv("STOCK_API_URL", "https://test.stock.local")
+    import importlib
+    from signal_v2 import config as cfg
+    importlib.reload(cfg)
+    from signal_v2 import main as main_mod
+    importlib.reload(main_mod)
+    with caplog.at_level(logging.WARNING, logger="signal_v2.main"):
+        with TestClient(main_mod.app) as client:
+            client.get("/health")
+        assert any("WEBAI_API_KEY" in rec.message for rec in caplog.records)