feat(signal_v2): foundation — config + state + requirements

- signal_v2/config.py: Settings dataclass loading web-ai/.env explicitly
- signal_v2/state.py: PollState dataclass + module-level singleton
- requirements.txt: httpx / fastapi / uvicorn / pytest-asyncio / respx
- .gitignore: signal_v2/data/*.db (WAL/SHM)
- empty tests/ marker

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

signal_v2/state.py

@@ -0,0 +1,15 @@
+"""PollState — process-wide singleton."""
+from dataclasses import dataclass, field
+
+
+@dataclass
+class PollState:
+    portfolio: dict | None = None
+    news_sentiment: dict | None = None
+    screener_preview: dict | None = None
+    last_updated: dict[str, str] = field(default_factory=dict)
+    fetch_errors: dict[str, int] = field(default_factory=dict)
+
+
+# Process-wide singleton. Phase 3 imports: from signal_v2.state import state
+state = PollState()