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>

requirements.txt

@@ -0,0 +1,8 @@
+# Signal V2 dependencies (added 2026-05-16, Phase 2)
+httpx>=0.27
+fastapi>=0.110
+uvicorn>=0.27
+python-dotenv>=1.0
+pytest>=8.0
+pytest-asyncio>=0.23
+respx>=0.21