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/agents/npc.go

@@ -0,0 +1,38 @@
+package agents
+
+import (
+	"fmt"
+
+	"mmgame/internal/scripts"
+)
+
+type NPCAgent struct {
+	Role       *scripts.Role
+	Background string
+	PublicInfo string
+	memory     []ChatMessage
+}
+
+func NewNPCAgent(role *scripts.Role, background, publicInfo string) *NPCAgent {
+	return &NPCAgent{Role: role, Background: background, PublicInfo: publicInfo}
+}
+
+func (a *NPCAgent) Respond(phaseName string, recentMessages []map[string]string, playerMessage string, isPrivate bool, knownClues []string, phaseGoal string, allRoles []*scripts.Role) map[string]interface{} {
+	if len(recentMessages) > 5 {
+		recentMessages = recentMessages[len(recentMessages)-5:]
+	}
+	var recent []string
+	for _, m := range recentMessages {
+		sender := m["sender_name"]
+		if sender == "" {
+			sender = m["sender"]
+		}
+		recent = append(recent, fmt.Sprintf("%s: %s", sender, m["content"]))
+	}
+	result := NPCGenerate(a.Role, a.Background, phaseName, recent, playerMessage, isPrivate, knownClues, phaseGoal, allRoles)
+	if content, ok := result["content"].(string); ok && content != "" {
+		a.memory = append(a.memory, ChatMessage{Role: "assistant", Content: content})
+		return result
+	}
+	return map[string]interface{}{"content": a.Role.Name + "陷入了沉思……", "action": "speak"}
+}