feat(backend): 授权改为时长兑换券体系 + 退役 ed25519/HMAC + 平台生码工具

- 新增 license_codes 码池表 + model.LicenseCode；licenses 加 tier 档位列
- LicenseService.Redeem：单事务 FOR UPDATE 校验码未用 → 时长叠加(可叠加，0=永久)
  → 写 type/tier/max_devices → 绑设备(超限整笔回滚) → 标记已用 → 即时失效 phase 缓存
  路由仍 POST /license/activate，客户端零破坏
- util.GenerateRedeemCode/NormalizeCode：JIUKU-XXXX-XXXX 短码(crypto/rand)
- cmd/gencode：平台批量生成兑换码并落库；删除 cmd/issue、cmd/genkey
- 退役 ed25519 + HMAC：删 util/license_key、GenerateKey、License 全部 config 字段
  及生产启动私钥校验；trial 改直接建行(无需私钥、去 Fatal)
- tier 档位钩子默认 standard，分档消费模式后续设计
- 测试：Redeem 全场景(叠加/过期重置/永久/一码一次/无效/设备上限回滚)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/handler/auth_test.go

@@ -11,10 +11,8 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
-	"github.com/wangjia/jiu/backend/config"
 	"github.com/wangjia/jiu/backend/internal/middleware"
 	"github.com/wangjia/jiu/backend/internal/service"
-	"github.com/wangjia/jiu/backend/internal/util"
 	"github.com/wangjia/jiu/backend/testutil"
 )
 
@@ -301,16 +299,13 @@ func TestAuthHandler_Login_ResponseContainsUserInfo(t *testing.T) {
 // #8 心跳 /auth/ping 回带授权概况，客户端据此免去单独轮询 /license/info。
 func TestAuthHandler_Ping_ReturnsLicense(t *testing.T) {
 	db := testutil.SetupTestDB()
-	priv, _, err := util.GenerateEd25519KeyPair()
-	require.NoError(t, err)
-	config.C.License.Ed25519PrivateKey = priv
 
 	shop := testutil.CreateTestShop(db, "AHPING")
 	testutil.CreateTestUser(db, shop.ID, "admin", "password123", "admin")
 
 	authSvc := service.NewAuthService(db)
 	// 登录触发首登自动 trial。
-	_, _, err = authSvc.Login("AHPING", "admin", "password123", service.DeviceInfo{Platform: "windows"})
+	_, _, err := authSvc.Login("AHPING", "admin", "password123", service.DeviceInfo{Platform: "windows"})
 	require.NoError(t, err)
 
 	licSvc := service.NewLicenseService(db)