chore(signal_v2): sync holidays.json from stock + start.bat launcher

- holidays.json: authoritative copy from web-backend/stock/app/holidays.json
  (replaces 13-date stub from Task 3; now 16 dates including Jan/May/Dec edges)
- start.bat: uvicorn launcher (cd to web-ai root, host 0.0.0.0, port 8001)

19 tests still pass.

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

signal_v2/start.bat

@@ -0,0 +1,3 @@
+@echo off
+cd /d "%~dp0\.."
+python -m uvicorn signal_v2.main:app --host 0.0.0.0 --port 8001