feat(admin): 添加管理员直接修改用户 API Key 分组的功能

- 新增 PUT /api/v1/admin/api-keys/:id 端点，允许管理员修改任意用户 API Key 的分组绑定
- 跳过用户级权限校验但保留分组有效性验证，修改后触发认证缓存失效
- Service 层支持三态语义：nil=不修改，0=解绑，>0=绑定，<0=拒绝
- 指针值拷贝保证安全隔离，负数 groupID 返回 400 INVALID_GROUP_ID
- 前端 UserApiKeysModal 新增可点击的分组选择下拉框，支持多 Key 并发更新
- 下拉支持视口翻转和滚动关闭，按钮有 disabled 和加载状态
- 覆盖：后端 20 个单元测试 (Service 11 + Handler 9) + 前端 16 个 E2E 测试
- golangci-lint 0 issues, make test-unit 全部通过

backend/internal/service/admin_service.go

@@ -9,6 +9,7 @@ import (
 	"strings"
 	"time"
 
+	infraerrors "github.com/Wei-Shaw/sub2api/internal/pkg/errors"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/httpclient"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/logger"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/pagination"
@@ -42,6 +43,9 @@ type AdminService interface {
 	GetGroupAPIKeys(ctx context.Context, groupID int64, page, pageSize int) ([]APIKey, int64, error)
 	UpdateGroupSortOrders(ctx context.Context, updates []GroupSortOrderUpdate) error
 
+	// API Key management (admin)
+	AdminUpdateAPIKeyGroupID(ctx context.Context, keyID int64, groupID *int64) (*APIKey, error)
+
 	// Account management
 	ListAccounts(ctx context.Context, page, pageSize int, platform, accountType, status, search string, groupID int64) ([]Account, int64, error)
 	GetAccount(ctx context.Context, id int64) (*Account, error)
@@ -1185,6 +1189,53 @@ func (s *adminServiceImpl) UpdateGroupSortOrders(ctx context.Context, updates []
 	return s.groupRepo.UpdateSortOrders(ctx, updates)
 }
 
+// AdminUpdateAPIKeyGroupID 管理员修改 API Key 分组绑定
+// groupID: nil=不修改, 指向0=解绑, 指向正整数=绑定到目标分组
+func (s *adminServiceImpl) AdminUpdateAPIKeyGroupID(ctx context.Context, keyID int64, groupID *int64) (*APIKey, error) {
+	apiKey, err := s.apiKeyRepo.GetByID(ctx, keyID)
+	if err != nil {
+		return nil, err
+	}
+
+	if groupID == nil {
+		// nil 表示不修改，直接返回
+		return apiKey, nil
+	}
+
+	if *groupID < 0 {
+		return nil, infraerrors.BadRequest("INVALID_GROUP_ID", "group_id must be non-negative")
+	}
+
+	if *groupID == 0 {
+		// 0 表示解绑分组
+		apiKey.GroupID = nil
+		apiKey.Group = nil
+	} else {
+		// 验证目标分组存在且状态为 active
+		group, err := s.groupRepo.GetByID(ctx, *groupID)
+		if err != nil {
+			return nil, err
+		}
+		if group.Status != StatusActive {
+			return nil, infraerrors.BadRequest("GROUP_NOT_ACTIVE", "target group is not active")
+		}
+		gid := *groupID
+		apiKey.GroupID = &gid
+		apiKey.Group = group
+	}
+
+	if err := s.apiKeyRepo.Update(ctx, apiKey); err != nil {
+		return nil, fmt.Errorf("update api key: %w", err)
+	}
+
+	// 失效认证缓存
+	if s.authCacheInvalidator != nil {
+		s.authCacheInvalidator.InvalidateAuthCacheByKey(ctx, apiKey.Key)
+	}
+
+	return apiKey, nil
+}
+
 // Account management implementations
 func (s *adminServiceImpl) ListAccounts(ctx context.Context, page, pageSize int, platform, accountType, status, search string, groupID int64) ([]Account, int64, error) {
 	params := pagination.PaginationParams{Page: page, PageSize: pageSize}