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/config/config.go

@@ -32,8 +32,9 @@ type JWTConfig struct {
 }
 
 type LicenseConfig struct {
-	HMACSecret      string `mapstructure:"hmac_secret"`       // legacy, kept for backward compat
-	Ed25519PublicKey string `mapstructure:"ed25519_public_key"` // base64 Ed25519 public key for token verification
+	HMACSecret       string `mapstructure:"hmac_secret"`        // legacy, kept for backward compat
+	Ed25519PublicKey  string `mapstructure:"ed25519_public_key"`  // base64 Ed25519 public key for token verification
+	Ed25519PrivateKey string `mapstructure:"ed25519_private_key"` // base64 Ed25519 private key for token signing (keep in Bitwarden)
 }
 
 type StorageConfig struct {
@@ -59,6 +60,7 @@ func Load() {
 	_ = viper.BindEnv("jwt.secret", "JWT_SECRET")
 	_ = viper.BindEnv("license.hmac_secret", "LICENSE_HMAC_SECRET")
 	_ = viper.BindEnv("license.ed25519_public_key", "LICENSE_ED25519_PUBLIC_KEY")
+	_ = viper.BindEnv("license.ed25519_private_key", "LICENSE_ED25519_PRIVATE_KEY")
 	_ = viper.BindEnv("storage.upload_dir", "STORAGE_UPLOAD_DIR")
 	_ = viper.BindEnv("storage.base_url", "STORAGE_BASE_URL")
 	_ = viper.BindEnv("storage.public_url", "STORAGE_PUBLIC_URL")