feat(admin): add super admin initialization endpoint

Add POST /api/v1/admin/setup-super-admin endpoint to initialize the first super admin.
The endpoint can only be used once - after a super admin exists, subsequent requests
are rejected. Requires valid setup_secret configured via APP_SETUP_SECRET environment
variable or setup_secret in config file.

internal/handler/setup_handler.go

@@ -0,0 +1,38 @@
+package handler
+
+import (
+	"with_you/internal/pkg/response"
+	"with_you/internal/service"
+
+	"github.com/gin-gonic/gin"
+)
+
+type SetupHandler struct {
+	setupService service.SetupService
+}
+
+func NewSetupHandler(setupService service.SetupService) *SetupHandler {
+	return &SetupHandler{
+		setupService: setupService,
+	}
+}
+
+func (h *SetupHandler) SetupSuperAdmin(c *gin.Context) {
+	type SetupSuperAdminRequest struct {
+		UserID      string `json:"user_id" binding:"required"`
+		SetupSecret string `json:"setup_secret" binding:"required"`
+	}
+
+	var req SetupSuperAdminRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		response.BadRequest(c, "user_id and setup_secret are required")
+		return
+	}
+
+	if err := h.setupService.SetupSuperAdmin(c.Request.Context(), req.UserID, req.SetupSecret); err != nil {
+		response.HandleError(c, err, "failed to setup super admin")
+		return
+	}
+
+	response.SuccessWithMessage(c, "超级管理员设置成功", nil)
+}