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/ws.go

@@ -0,0 +1,121 @@
+package handlers
+
+import (
+	"encoding/json"
+	"net/http"
+	"sync"
+
+	"github.com/go-chi/chi/v5"
+	"github.com/gorilla/websocket"
+)
+
+type Client struct {
+	Conn   *websocket.Conn
+	RoleID string
+}
+
+var (
+	connsMu     sync.RWMutex
+	activeConns = map[string][]*Client{}
+)
+
+var upgrader = websocket.Upgrader{
+	CheckOrigin: func(r *http.Request) bool { return true },
+}
+
+func removeConn(gameID string, c *Client) {
+	connsMu.Lock()
+	defer connsMu.Unlock()
+	clients := activeConns[gameID]
+	for i, x := range clients {
+		if x == c {
+			activeConns[gameID] = append(clients[:i], clients[i+1:]...)
+			break
+		}
+	}
+	if len(activeConns[gameID]) == 0 {
+		delete(activeConns, gameID)
+	}
+}
+
+func removeGameConns(gameID string) {
+	connsMu.Lock()
+	defer connsMu.Unlock()
+	if clients, ok := activeConns[gameID]; ok {
+		for _, c := range clients {
+			_ = c.Conn.Close()
+		}
+	}
+	delete(activeConns, gameID)
+}
+
+func broadcast(gameID string, message interface{}) {
+	data, err := json.Marshal(message)
+	if err != nil {
+		return
+	}
+	connsMu.RLock()
+	clients := append([]*Client{}, activeConns[gameID]...)
+	connsMu.RUnlock()
+	for _, c := range clients {
+		if err := c.Conn.WriteMessage(websocket.TextMessage, data); err != nil {
+			removeConn(gameID, c)
+		}
+	}
+}
+
+func broadcastToRole(gameID, targetRoleID string, message interface{}) {
+	data, err := json.Marshal(message)
+	if err != nil {
+		return
+	}
+	connsMu.RLock()
+	clients := append([]*Client{}, activeConns[gameID]...)
+	connsMu.RUnlock()
+	for _, c := range clients {
+		if c.RoleID != "" && c.RoleID != targetRoleID {
+			continue
+		}
+		if err := c.Conn.WriteMessage(websocket.TextMessage, data); err != nil {
+			removeConn(gameID, c)
+		}
+	}
+}
+
+func GameWebSocket(w http.ResponseWriter, r *http.Request) {
+	gameID := chi.URLParam(r, "game_id")
+	roleID := r.URL.Query().Get("role_id")
+
+	conn, err := upgrader.Upgrade(w, r, nil)
+	if err != nil {
+		return
+	}
+	client := &Client{Conn: conn, RoleID: roleID}
+
+	connsMu.Lock()
+	activeConns[gameID] = append(activeConns[gameID], client)
+	connsMu.Unlock()
+
+	defer func() {
+		removeConn(gameID, client)
+		_ = conn.Close()
+	}()
+
+	for {
+		_, data, err := conn.ReadMessage()
+		if err != nil {
+			return
+		}
+		var msg struct {
+			Type string `json:"type"`
+		}
+		if err := json.Unmarshal(data, &msg); err != nil {
+			continue
+		}
+		if msg.Type == "ping" {
+			if err := conn.WriteMessage(websocket.TextMessage, []byte(`{"type":"pong"}`)); err != nil {
+				return
+			}
+		}
+	}
+}