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.

backend/.env.example

@@ -0,0 +1,13 @@
+DATABASE_URL=sqlite+aiosqlite:///./mmgame.db
+SECRET_KEY=mmgame-dev-secret-key-change-in-production
+
+AI_BACKEND=rule
+# AI_BACKEND=deepseek
+# AI_BACKEND=ollama
+
+DEEPSEEK_API_KEY=
+DEEPSEEK_BASE_URL=https://api.deepseek.com
+DEEPSEEK_MODEL=deepseek-chat
+
+OLLAMA_BASE_URL=http://localhost:11434
+OLLAMA_MODEL=qwen2.5:7b