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>

.gitignore

@@ -59,3 +59,7 @@ Desktop.ini
 KIS_SETUP.md
 # Claude Code subagent state
 .claude/
+
+# Signal V2 runtime data
+signal_v2/data/*.db
+signal_v2/data/*.db-*