feat: LicenseGuard 中间件 + Flutter license model/repo (21D+21E)

21D — 后端:
- Claims 新增 LicenseExpiresAt (*int64 unix 秒)，写入 JWT 避免每次查库
- middleware/license_guard.go: CalcLicensePhase / LicenseGuard / GetLicensePhase
  - grace(0-7d): 允许通行
  - readonly(7-15d): 拦截非 GET 写操作 → 403
  - locked(15d+): 全部拦截 → 403
- auth.go: issueTokens 在 JWT 中嵌入 license expires_at；Login 检查 locked 拒绝登录
- router: license/* 路由豁免 LicenseGuard（锁定时仍可激活/查状态）

21E — Flutter 前端:
- models/license.dart: 新 LicenseInfo，含 phase/maxDevices，去掉旧 activatedAt
- core/device/device_id.dart: 持久化 UUID-v4 作为设备 ID（SharedPreferences）
- repositories/license_repository.dart: getInfo/activate/deactivate，激活时携带设备信息
- providers/license_provider.dart: 改接 LicenseRepository
- settings_screen.dart: activatedAt 改为显示 maxDevices

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/middleware/auth.go

@@ -10,16 +10,18 @@ import (
 )
 
 type Claims struct {
-	UserID uint64 `json:"user_id"`
-	ShopID uint64 `json:"shop_id"`
-	Role   string `json:"role"`
+	UserID            uint64 `json:"user_id"`
+	ShopID            uint64 `json:"shop_id"`
+	Role              string `json:"role"`
+	LicenseExpiresAt  *int64 `json:"lic_exp,omitempty"` // unix seconds; nil = perpetual
 	jwt.RegisteredClaims
 }
 
 const (
-	CtxUserID = "user_id"
-	CtxShopID = "shop_id"
-	CtxRole   = "role"
+	CtxUserID           = "user_id"
+	CtxShopID           = "shop_id"
+	CtxRole             = "role"
+	CtxLicenseExpiresAt = "lic_exp"
 )
 
 func JWT() gin.HandlerFunc {
@@ -43,6 +45,7 @@ func JWT() gin.HandlerFunc {
 		c.Set(CtxUserID, claims.UserID)
 		c.Set(CtxShopID, claims.ShopID)
 		c.Set(CtxRole, claims.Role)
+		c.Set(CtxLicenseExpiresAt, claims.LicenseExpiresAt)
 		c.Next()
 	}
 }