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/model/license.go

@@ -4,12 +4,14 @@ import "time"
 
 type License struct {
 	Base
-	ShopID      uint64     `gorm:"not null;index" json:"shop_id"`
-	LicenseKey  string     `gorm:"size:255;uniqueIndex" json:"license_key"`
-	DeviceID    string     `gorm:"size:255" json:"device_id"`
-	Type        string     `gorm:"type:enum('trial','monthly','annual','lifetime');default:'trial'" json:"type"`
-	ExpiresAt   *time.Time `json:"expires_at"`
-	IsActive    bool       `gorm:"default:true" json:"is_active"`
-	Features    JSON       `gorm:"type:json" json:"features,omitempty"`
-	ActivatedAt *time.Time `json:"activated_at"`
+	ShopID     uint64     `gorm:"not null;index" json:"shop_id"`
+	LicenseKey string     `gorm:"size:2048;uniqueIndex" json:"license_key"`
+	Type       string     `gorm:"type:enum('trial','monthly','annual','lifetime');default:'trial'" json:"type"`
+	ExpiresAt  *time.Time `json:"expires_at"`
+	IsActive   bool       `gorm:"default:true" json:"is_active"`
+	MaxDevices int        `gorm:"default:3" json:"max_devices"`
+	Features   JSON       `gorm:"type:json" json:"features,omitempty"`
+	// Deprecated: device binding is now tracked in license_devices table.
+	DeviceID    string     `gorm:"size:255" json:"device_id,omitempty"`
+	ActivatedAt *time.Time `json:"activated_at,omitempty"`
 }

backend/internal/model/license_device.go

@@ -0,0 +1,16 @@
+package model
+
+import "time"
+
+type LicenseDevice struct {
+	ID          uint64     `gorm:"primaryKey;autoIncrement" json:"id"`
+	LicenseID   uint64     `gorm:"not null;index" json:"license_id"`
+	ShopID      uint64     `gorm:"not null;index" json:"shop_id"`
+	DeviceID    string     `gorm:"size:255;not null" json:"device_id"`
+	DeviceName  string     `gorm:"size:255" json:"device_name"`
+	Platform    string     `gorm:"size:50" json:"platform"` // windows|macos|android|ios|web
+	ActivatedAt time.Time  `gorm:"not null;autoCreateTime" json:"activated_at"`
+	LastSeenAt  time.Time  `gorm:"not null;autoUpdateTime" json:"last_seen_at"`
+}
+
+func (LicenseDevice) TableName() string { return "license_devices" }