feat(backend): license_devices 表 + 注册自动签发 trial (21B+21C)

21B — DB & model:
- licenses: 扩展 license_key 为 2048 字节（容纳 Ed25519 JWT），
  新增 max_devices INT DEFAULT 3，device_id/activated_at 标为 deprecated
- 新增 license_devices 表（license_id+device_id 唯一索引）
- model/license_device.go：LicenseDevice struct
- main.go AutoMigrate 加入 LicenseDevice
- testutil/setup.go 同步 SQLite DDL

21C — trial at register:
- config: 新增 Ed25519PrivateKey 配置项（LICENSE_ED25519_PRIVATE_KEY 环境变量）
- service/license.go: createTrialLicense(tx, shopID) — 签发 30d trial，
  私钥未配置时静默跳过（开发/测试不影响）
- service/auth.go: Register 事务末尾调用 createTrialLicense

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

backend/internal/service/auth.go

@@ -123,6 +123,8 @@ func (s *AuthService) Register(in RegisterInput) (*RegisterResult, error) {
 			return err
 		}
 
+		createTrialLicense(tx, shop.ID)
+
 		result = RegisterResult{
 			ShopCode: shop.Code,
 			ShopName: shop.Name,

backend/internal/service/license.go

@@ -6,6 +6,7 @@ import (
 	"encoding/base32"
 	"errors"
 	"fmt"
+	"log"
 	"strings"
 	"time"
 
@@ -13,6 +14,7 @@ import (
 
 	"github.com/wangjia/jiu/backend/config"
 	"github.com/wangjia/jiu/backend/internal/model"
+	"github.com/wangjia/jiu/backend/internal/util"
 )
 
 var (
@@ -98,3 +100,40 @@ func (s *LicenseService) Deactivate(shopID uint64, deviceID string) error {
 		Where("shop_id = ? AND device_id = ?", shopID, deviceID).
 		Updates(map[string]interface{}{"device_id": "", "activated_at": nil}).Error
 }
+
+// createTrialLicense 在注册事务中为新门店签发 30 天 trial license。
+// 若私钥未配置则静默跳过（开发/测试环境可不配置私钥）。
+func createTrialLicense(tx *gorm.DB, shopID uint64) {
+	privKey := config.C.License.Ed25519PrivateKey
+	if privKey == "" {
+		log.Printf("[license] Ed25519 private key not configured, skipping trial for shop %d", shopID)
+		return
+	}
+
+	expiresAt := time.Now().Add(30 * 24 * time.Hour)
+	expiresUnix := expiresAt.Unix()
+	payload := util.LicensePayload{
+		ShopID:     shopID,
+		Type:       "trial",
+		IssuedAt:   time.Now().Unix(),
+		ExpiresAt:  &expiresUnix,
+		MaxDevices: 1,
+	}
+	token, err := util.IssueLicenseToken(payload, privKey)
+	if err != nil {
+		log.Printf("[license] failed to issue trial token for shop %d: %v", shopID, err)
+		return
+	}
+
+	lic := model.License{
+		ShopID:     shopID,
+		LicenseKey: token,
+		Type:       "trial",
+		ExpiresAt:  &expiresAt,
+		IsActive:   true,
+		MaxDevices: 1,
+	}
+	if err := tx.Create(&lic).Error; err != nil {
+		log.Printf("[license] failed to create trial license for shop %d: %v", shopID, err)
+	}
+}