优化 API 密钥管理与展示

- 每个商户限制最多 5 个 API 密钥，超限拒绝创建并提示
- API 密钥权限列改用中文标签展示，新增按用途命名提示
- 优化 API 密钥表格列宽，App Key 完整单行展示
- 补充密钥数量上限测试

backend/internal/service/merchant_test.go

@@ -0,0 +1,37 @@
+package service
+
+import (
+	"fmt"
+	"strings"
+	"testing"
+
+	"affiliate_dash/internal/model"
+)
+
+func TestCreateAPIClientEnforcesPerMerchantLimit(t *testing.T) {
+	db := newServiceTestDB(t)
+	codec, err := NewSecretCodec("test-master-key")
+	if err != nil {
+		t.Fatalf("codec: %v", err)
+	}
+	svc := NewMerchantService(db, codec, NewTenantService(db))
+	merchant := model.Merchant{Code: "merchant-api-limit", Name: "限数商户", Status: model.MerchantStatusActive}
+	if err := db.Create(&merchant).Error; err != nil {
+		t.Fatalf("create merchant: %v", err)
+	}
+	for i := 0; i < MaxAPIClientsPerMerchant; i++ {
+		if _, err := svc.CreateAPIClient(merchant.ID, CreateAPIClientInput{
+			Name:   fmt.Sprintf("key-%d", i),
+			Scopes: "products:read",
+		}, 1); err != nil {
+			t.Fatalf("create client %d: %v", i, err)
+		}
+	}
+	_, err = svc.CreateAPIClient(merchant.ID, CreateAPIClientInput{
+		Name:   "overflow",
+		Scopes: "products:read",
+	}, 1)
+	if err == nil || !strings.Contains(err.Error(), fmt.Sprintf("最多可创建 %d 个", MaxAPIClientsPerMerchant)) {
+		t.Fatalf("expected per-merchant limit error, got %v", err)
+	}
+}