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/pull_worker.py

@@ -0,0 +1,58 @@
+"""Polling loop — async cron + state update."""
+from __future__ import annotations
+import asyncio
+import logging
+from datetime import datetime
+
+from signal_v2.scheduler import (
+    KST, _is_market_day, _is_polling_window, _next_interval,
+)
+from signal_v2.state import PollState
+from signal_v2.stock_client import StockClient
+
+logger = logging.getLogger(__name__)
+
+
+async def poll_loop(
+    client: StockClient, state: PollState, shutdown: asyncio.Event
+) -> None:
+    """FastAPI lifespan 에서 asyncio.create_task 로 시작."""
+    logger.info("poll_loop started")
+    while not shutdown.is_set():
+        now = datetime.now(KST)
+        if _is_market_day(now) and _is_polling_window(now):
+            try:
+                await _run_polling_cycle(client, state)
+            except Exception:
+                logger.exception("poll cycle failed")
+        interval = _next_interval(now)
+        try:
+            await asyncio.wait_for(shutdown.wait(), timeout=interval)
+            break
+        except asyncio.TimeoutError:
+            continue
+    logger.info("poll_loop ended")
+
+
+async def _run_polling_cycle(client: StockClient, state: PollState) -> None:
+    """3 endpoint 병렬 fetch + state 갱신."""
+    portfolio, sentiment, screener = await asyncio.gather(
+        client.get_portfolio(),
+        client.get_news_sentiment(),
+        client.run_screener_preview(),
+        return_exceptions=True,
+    )
+    now_iso = datetime.now(KST).isoformat()
+
+    for name, result in (
+        ("portfolio", portfolio),
+        ("news_sentiment", sentiment),
+        ("screener_preview", screener),
+    ):
+        if isinstance(result, dict):
+            setattr(state, name, result)
+            state.last_updated[name] = now_iso
+            state.fetch_errors[name] = 0
+        else:
+            state.fetch_errors[name] = state.fetch_errors.get(name, 0) + 1
+            logger.warning("fetch %s failed: %r", name, result)