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/internal/handlers/scripts.go

@@ -0,0 +1,36 @@
+package handlers
+
+import (
+	"net/http"
+
+	"github.com/go-chi/chi/v5"
+
+	"mmgame/internal/scripts"
+)
+
+func ListScripts(w http.ResponseWriter, r *http.Request) {
+	items := []map[string]interface{}{}
+	for _, s := range scripts.SCRIPTS {
+		items = append(items, map[string]interface{}{
+			"id":          s.ID,
+			"title":       s.Title,
+			"type":        s.Type,
+			"difficulty":  s.Difficulty,
+			"min_players": s.PlayerCount.Min,
+			"max_players": s.PlayerCount.Max,
+			"duration":    s.Duration,
+			"background":  s.Background,
+		})
+	}
+	writeJSON(w, 200, items)
+}
+
+func GetScript(w http.ResponseWriter, r *http.Request) {
+	scriptID := chi.URLParam(r, "script_id")
+	s := scripts.GetByID(scriptID)
+	if s == nil {
+		writeError(w, 404, "剧本不存在")
+		return
+	}
+	writeJSON(w, 200, s)
+}