feat(groups): add rate multipliers management modal

Add a dedicated modal in group management for viewing, adding, editing,
and deleting per-user rate multipliers within a group.

Backend:
- GET /admin/groups/:id/rate-multipliers - list entries with user details
- PUT /admin/groups/:id/rate-multipliers - batch sync (full replace)
- DELETE /admin/groups/:id/rate-multipliers - clear all entries
- Repository: GetByGroupID, SyncGroupRateMultipliers methods on
  user_group_rate_multipliers table (same table as user-side rates)

Frontend:
- New GroupRateMultipliersModal component with:
  - User search and add with email autocomplete
  - Editable rate column with local edit mode (cancel/save)
  - Batch adjust: multiply all rates by a factor
  - Clear all (local operation, requires save to persist)
  - Pagination (10/20/50 per page)
  - Platform icon with brand colors in group info bar
  - Unsaved changes indicator with revert option
- Unit tests for all three backend endpoints

backend/internal/service/admin_service.go

@@ -42,6 +42,9 @@ type AdminService interface {
 	UpdateGroup(ctx context.Context, id int64, input *UpdateGroupInput) (*Group, error)
 	DeleteGroup(ctx context.Context, id int64) error
 	GetGroupAPIKeys(ctx context.Context, groupID int64, page, pageSize int) ([]APIKey, int64, error)
+	GetGroupRateMultipliers(ctx context.Context, groupID int64) ([]UserGroupRateEntry, error)
+	ClearGroupRateMultipliers(ctx context.Context, groupID int64) error
+	BatchSetGroupRateMultipliers(ctx context.Context, groupID int64, entries []GroupRateMultiplierInput) error
 	UpdateGroupSortOrders(ctx context.Context, updates []GroupSortOrderUpdate) error
 
 	// API Key management (admin)
@@ -1244,6 +1247,27 @@ func (s *adminServiceImpl) GetGroupAPIKeys(ctx context.Context, groupID int64, p
 	return keys, result.Total, nil
 }
 
+func (s *adminServiceImpl) GetGroupRateMultipliers(ctx context.Context, groupID int64) ([]UserGroupRateEntry, error) {
+	if s.userGroupRateRepo == nil {
+		return nil, nil
+	}
+	return s.userGroupRateRepo.GetByGroupID(ctx, groupID)
+}
+
+func (s *adminServiceImpl) ClearGroupRateMultipliers(ctx context.Context, groupID int64) error {
+	if s.userGroupRateRepo == nil {
+		return nil
+	}
+	return s.userGroupRateRepo.DeleteByGroupID(ctx, groupID)
+}
+
+func (s *adminServiceImpl) BatchSetGroupRateMultipliers(ctx context.Context, groupID int64, entries []GroupRateMultiplierInput) error {
+	if s.userGroupRateRepo == nil {
+		return nil
+	}
+	return s.userGroupRateRepo.SyncGroupRateMultipliers(ctx, groupID, entries)
+}
+
 func (s *adminServiceImpl) UpdateGroupSortOrders(ctx context.Context, updates []GroupSortOrderUpdate) error {
 	return s.groupRepo.UpdateSortOrders(ctx, updates)
 }

backend/internal/service/admin_service_group_rate_test.go

@@ -0,0 +1,176 @@
+//go:build unit
+
+package service
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// userGroupRateRepoStubForGroupRate implements UserGroupRateRepository for group rate tests.
+type userGroupRateRepoStubForGroupRate struct {
+	getByGroupIDData map[int64][]UserGroupRateEntry
+	getByGroupIDErr  error
+
+	deletedGroupIDs  []int64
+	deleteByGroupErr error
+
+	syncedGroupID int64
+	syncedEntries []GroupRateMultiplierInput
+	syncGroupErr  error
+}
+
+func (s *userGroupRateRepoStubForGroupRate) GetByUserID(_ context.Context, _ int64) (map[int64]float64, error) {
+	panic("unexpected GetByUserID call")
+}
+
+func (s *userGroupRateRepoStubForGroupRate) GetByUserAndGroup(_ context.Context, _, _ int64) (*float64, error) {
+	panic("unexpected GetByUserAndGroup call")
+}
+
+func (s *userGroupRateRepoStubForGroupRate) GetByGroupID(_ context.Context, groupID int64) ([]UserGroupRateEntry, error) {
+	if s.getByGroupIDErr != nil {
+		return nil, s.getByGroupIDErr
+	}
+	return s.getByGroupIDData[groupID], nil
+}
+
+func (s *userGroupRateRepoStubForGroupRate) SyncUserGroupRates(_ context.Context, _ int64, _ map[int64]*float64) error {
+	panic("unexpected SyncUserGroupRates call")
+}
+
+func (s *userGroupRateRepoStubForGroupRate) SyncGroupRateMultipliers(_ context.Context, groupID int64, entries []GroupRateMultiplierInput) error {
+	s.syncedGroupID = groupID
+	s.syncedEntries = entries
+	return s.syncGroupErr
+}
+
+func (s *userGroupRateRepoStubForGroupRate) DeleteByGroupID(_ context.Context, groupID int64) error {
+	s.deletedGroupIDs = append(s.deletedGroupIDs, groupID)
+	return s.deleteByGroupErr
+}
+
+func (s *userGroupRateRepoStubForGroupRate) DeleteByUserID(_ context.Context, _ int64) error {
+	panic("unexpected DeleteByUserID call")
+}
+
+func TestAdminService_GetGroupRateMultipliers(t *testing.T) {
+	t.Run("returns entries for group", func(t *testing.T) {
+		repo := &userGroupRateRepoStubForGroupRate{
+			getByGroupIDData: map[int64][]UserGroupRateEntry{
+				10: {
+					{UserID: 1, UserName: "alice", UserEmail: "alice@test.com", RateMultiplier: 1.5},
+					{UserID: 2, UserName: "bob", UserEmail: "bob@test.com", RateMultiplier: 0.8},
+				},
+			},
+		}
+		svc := &adminServiceImpl{userGroupRateRepo: repo}
+
+		entries, err := svc.GetGroupRateMultipliers(context.Background(), 10)
+		require.NoError(t, err)
+		require.Len(t, entries, 2)
+		require.Equal(t, int64(1), entries[0].UserID)
+		require.Equal(t, "alice", entries[0].UserName)
+		require.Equal(t, 1.5, entries[0].RateMultiplier)
+		require.Equal(t, int64(2), entries[1].UserID)
+		require.Equal(t, 0.8, entries[1].RateMultiplier)
+	})
+
+	t.Run("returns nil when repo is nil", func(t *testing.T) {
+		svc := &adminServiceImpl{userGroupRateRepo: nil}
+
+		entries, err := svc.GetGroupRateMultipliers(context.Background(), 10)
+		require.NoError(t, err)
+		require.Nil(t, entries)
+	})
+
+	t.Run("returns empty slice for group with no entries", func(t *testing.T) {
+		repo := &userGroupRateRepoStubForGroupRate{
+			getByGroupIDData: map[int64][]UserGroupRateEntry{},
+		}
+		svc := &adminServiceImpl{userGroupRateRepo: repo}
+
+		entries, err := svc.GetGroupRateMultipliers(context.Background(), 99)
+		require.NoError(t, err)
+		require.Nil(t, entries)
+	})
+
+	t.Run("propagates repo error", func(t *testing.T) {
+		repo := &userGroupRateRepoStubForGroupRate{
+			getByGroupIDErr: errors.New("db error"),
+		}
+		svc := &adminServiceImpl{userGroupRateRepo: repo}
+
+		_, err := svc.GetGroupRateMultipliers(context.Background(), 10)
+		require.Error(t, err)
+		require.Contains(t, err.Error(), "db error")
+	})
+}
+
+func TestAdminService_ClearGroupRateMultipliers(t *testing.T) {
+	t.Run("deletes by group ID", func(t *testing.T) {
+		repo := &userGroupRateRepoStubForGroupRate{}
+		svc := &adminServiceImpl{userGroupRateRepo: repo}
+
+		err := svc.ClearGroupRateMultipliers(context.Background(), 42)
+		require.NoError(t, err)
+		require.Equal(t, []int64{42}, repo.deletedGroupIDs)
+	})
+
+	t.Run("returns nil when repo is nil", func(t *testing.T) {
+		svc := &adminServiceImpl{userGroupRateRepo: nil}
+
+		err := svc.ClearGroupRateMultipliers(context.Background(), 42)
+		require.NoError(t, err)
+	})
+
+	t.Run("propagates repo error", func(t *testing.T) {
+		repo := &userGroupRateRepoStubForGroupRate{
+			deleteByGroupErr: errors.New("delete failed"),
+		}
+		svc := &adminServiceImpl{userGroupRateRepo: repo}
+
+		err := svc.ClearGroupRateMultipliers(context.Background(), 42)
+		require.Error(t, err)
+		require.Contains(t, err.Error(), "delete failed")
+	})
+}
+
+func TestAdminService_BatchSetGroupRateMultipliers(t *testing.T) {
+	t.Run("syncs entries to repo", func(t *testing.T) {
+		repo := &userGroupRateRepoStubForGroupRate{}
+		svc := &adminServiceImpl{userGroupRateRepo: repo}
+
+		entries := []GroupRateMultiplierInput{
+			{UserID: 1, RateMultiplier: 1.5},
+			{UserID: 2, RateMultiplier: 0.8},
+		}
+		err := svc.BatchSetGroupRateMultipliers(context.Background(), 10, entries)
+		require.NoError(t, err)
+		require.Equal(t, int64(10), repo.syncedGroupID)
+		require.Equal(t, entries, repo.syncedEntries)
+	})
+
+	t.Run("returns nil when repo is nil", func(t *testing.T) {
+		svc := &adminServiceImpl{userGroupRateRepo: nil}
+
+		err := svc.BatchSetGroupRateMultipliers(context.Background(), 10, nil)
+		require.NoError(t, err)
+	})
+
+	t.Run("propagates repo error", func(t *testing.T) {
+		repo := &userGroupRateRepoStubForGroupRate{
+			syncGroupErr: errors.New("sync failed"),
+		}
+		svc := &adminServiceImpl{userGroupRateRepo: repo}
+
+		err := svc.BatchSetGroupRateMultipliers(context.Background(), 10, []GroupRateMultiplierInput{
+			{UserID: 1, RateMultiplier: 1.0},
+		})
+		require.Error(t, err)
+		require.Contains(t, err.Error(), "sync failed")
+	})
+}

backend/internal/service/admin_service_list_users_test.go

@@ -68,7 +68,15 @@ func (s *userGroupRateRepoStubForListUsers) SyncUserGroupRates(_ context.Context
 	panic("unexpected SyncUserGroupRates call")
 }
 
-func (s *userGroupRateRepoStubForListUsers) DeleteByGroupID(_ context.Context, groupID int64) error {
+func (s *userGroupRateRepoStubForListUsers) GetByGroupID(_ context.Context, _ int64) ([]UserGroupRateEntry, error) {
+	panic("unexpected GetByGroupID call")
+}
+
+func (s *userGroupRateRepoStubForListUsers) SyncGroupRateMultipliers(_ context.Context, _ int64, _ []GroupRateMultiplierInput) error {
+	panic("unexpected SyncGroupRateMultipliers call")
+}
+
+func (s *userGroupRateRepoStubForListUsers) DeleteByGroupID(_ context.Context, _ int64) error {
 	panic("unexpected DeleteByGroupID call")
 }
 

backend/internal/service/user_group_rate.go

@@ -2,6 +2,22 @@ package service
 
 import "context"
 
+// UserGroupRateEntry 分组下用户专属倍率条目
+type UserGroupRateEntry struct {
+	UserID         int64   `json:"user_id"`
+	UserName       string  `json:"user_name"`
+	UserEmail      string  `json:"user_email"`
+	UserNotes      string  `json:"user_notes"`
+	UserStatus     string  `json:"user_status"`
+	RateMultiplier float64 `json:"rate_multiplier"`
+}
+
+// GroupRateMultiplierInput 批量设置分组倍率的输入条目
+type GroupRateMultiplierInput struct {
+	UserID         int64   `json:"user_id"`
+	RateMultiplier float64 `json:"rate_multiplier"`
+}
+
 // UserGroupRateRepository 用户专属分组倍率仓储接口
 // 允许管理员为特定用户设置分组的专属计费倍率，覆盖分组默认倍率
 type UserGroupRateRepository interface {
@@ -13,10 +29,16 @@ type UserGroupRateRepository interface {
 	// 如果未设置专属倍率，返回 nil
 	GetByUserAndGroup(ctx context.Context, userID, groupID int64) (*float64, error)
 
+	// GetByGroupID 获取指定分组下所有用户的专属倍率
+	GetByGroupID(ctx context.Context, groupID int64) ([]UserGroupRateEntry, error)
+
 	// SyncUserGroupRates 同步用户的分组专属倍率
 	// rates: map[groupID]*rateMultiplier，nil 表示删除该分组的专属倍率
 	SyncUserGroupRates(ctx context.Context, userID int64, rates map[int64]*float64) error
 
+	// SyncGroupRateMultipliers 批量同步分组的用户专属倍率（替换整组数据）
+	SyncGroupRateMultipliers(ctx context.Context, groupID int64, entries []GroupRateMultiplierInput) error
+
 	// DeleteByGroupID 删除指定分组的所有用户专属倍率（分组删除时调用）
 	DeleteByGroupID(ctx context.Context, groupID int64) error