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/dm.go

@@ -0,0 +1,38 @@
+package agents
+
+import (
+	"fmt"
+
+	"mmgame/internal/scripts"
+)
+
+type DMAgent struct {
+	Script            *scripts.Script
+	currentPhaseIndex int
+	releasedClues     []string
+}
+
+func NewDMAgent(script *scripts.Script) *DMAgent {
+	return &DMAgent{Script: script}
+}
+
+func (a *DMAgent) GetOpening() string {
+	return fmt.Sprintf("欢迎来到【%s】。%s", a.Script.Title, a.Script.Background)
+}
+
+func (a *DMAgent) Act(phaseIndex int, playerProgress, context string) map[string]interface{} {
+	result := DMGenerate(a.Script, phaseIndex, a.releasedClues, context)
+	if len(result) > 0 {
+		return result
+	}
+	phases := a.Script.Phases
+	phase := &phases[len(phases)-1]
+	if phaseIndex < len(phases) {
+		phase = &phases[phaseIndex]
+	}
+	return map[string]interface{}{"action": "narrate", "content": "【" + phase.Name + "】" + phase.Description}
+}
+
+func (a *DMAgent) GetReplaySummary() string {
+	return fmt.Sprintf("剧本：%s\n真相：%s", a.Script.Title, a.Script.Truth)
+}