feat(backend): 首次登录无有效授权时自动发放 30 天试用

未激活门店（seed/手动建店/老数据，无任何 license 行）此前被当作永久授权，
既不拦截也无提示。现在登录时若门店无有效授权，自动签发 30 天 trial，
之后照常走 grace/readonly/locked 降级链。须在 issueTokens 之前发放，
使 JWT 的 lic_exp 带上新到期日。幂等（已有有效授权不重发），签发失败
仅记日志不阻断登录。复用注册路径的发放逻辑（抽出 issueTrialLicense）。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y2Wdwo7SmgBJU37cBrkhPK

backend/internal/service/auth_test.go

@@ -2,10 +2,14 @@ package service
 
 import (
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
+	"github.com/wangjia/jiu/backend/config"
+	"github.com/wangjia/jiu/backend/internal/model"
+	"github.com/wangjia/jiu/backend/internal/util"
 	"github.com/wangjia/jiu/backend/testutil"
 )
 
@@ -25,6 +29,42 @@ func TestAuthService_Login_Success(t *testing.T) {
 	assert.Equal(t, "admin", user.Username)
 }
 
+func TestAuthService_Login_AutoTrialOnFirstUse(t *testing.T) {
+	db := testutil.SetupTestDB()
+	priv, _, err := util.GenerateEd25519KeyPair()
+	require.NoError(t, err)
+	config.C.License.Ed25519PrivateKey = priv
+
+	shop := testutil.CreateTestShop(db, "TRIAL001")
+	testutil.CreateTestUser(db, shop.ID, "admin", "password123", "admin")
+
+	// 前置：门店「未激活」——无任何授权行
+	var before int64
+	db.Model(&model.License{}).Where("shop_id = ?", shop.ID).Count(&before)
+	require.EqualValues(t, 0, before)
+
+	svc := NewAuthService(db)
+
+	// 首次登录 → 自动签发 30 天 trial
+	_, _, err = svc.Login("TRIAL001", "admin", "password123", DeviceInfo{Platform: "windows"})
+	require.NoError(t, err)
+
+	var lic model.License
+	require.NoError(t, db.Where("shop_id = ? AND is_active = 1", shop.ID).First(&lic).Error)
+	assert.Equal(t, "trial", lic.Type)
+	assert.True(t, lic.IsActive)
+	require.NotNil(t, lic.ExpiresAt)
+	days := time.Until(*lic.ExpiresAt).Hours() / 24
+	assert.InDelta(t, 30, days, 1, "试用期应约为 30 天")
+
+	// 再次登录不重复发放
+	_, _, err = svc.Login("TRIAL001", "admin", "password123", DeviceInfo{Platform: "windows"})
+	require.NoError(t, err)
+	var count int64
+	db.Model(&model.License{}).Where("shop_id = ?", shop.ID).Count(&count)
+	assert.EqualValues(t, 1, count, "已有有效授权时不应再发放 trial")
+}
+
 func TestAuthService_Login_WrongPassword(t *testing.T) {
 	db := testutil.SetupTestDB()
 	shop := testutil.CreateTestShop(db, "HOTEL002")