refactor: rewrite backend in Go, replacing Python FastAPI

Port the FastAPI backend to Go 1.26, preserving all API functionality:
- auth (register/login/guest/logout/profile/history) with JWT + bcrypt
- scripts (list/detail) backed by embedded sample_data.json
- games (create/join/start/chat/phase/clue/vote/state/replay) + WebSocket
- rule-based NPC agent plus DeepSeek/Ollama AI backends
- SQLite persistence via modernc.org/sqlite (pure Go, no cgo)

Frontend and documents are unchanged.

start.bat

@@ -1,5 +1,5 @@
 @echo off
-title AI Murder Mystery MVP
+title AI Murder Mystery MVP (Go Backend)
 
 echo ========================================
 echo   AI Murder Mystery MVP - Launcher
@@ -8,8 +8,8 @@ echo.
 
 set "ROOT=%~dp0"
 
-echo [1/2] Starting backend (port 8000)...
-start "MM-Backend" /D "%ROOT%backend" uvicorn app.main:app --reload --port 8000
+echo [1/2] Starting Go backend (port 8000)...
+start "MM-Backend" /D "%ROOT%backend" go run .
 
 ping 127.0.0.1 -n 3 >nul
 
@@ -26,4 +26,4 @@ echo   Frontend: http://localhost:5173
 echo ========================================
 
 start http://localhost:5173
-pause
+pause