feat(channels): add "Available Channels" aggregate view

Add a read-only aggregate view per channel: its linked groups and a
deterministic wildcard-free supported-model list with pricing details.

Backend
- service.Channel.SupportedModels(): combine ModelMapping keys with
  same-platform ModelPricing.Models; trailing "*" keys expand via
  pricing prefix match; platforms without a mapping produce no
  entries (intentional "no mapping = not shown" rule).
- Extract splitWildcardSuffix() shared with toModelEntry.
- Build a per-call pricing lookup map (platform+lowerName -> *pricing)
  to avoid O(N*M) scans in SupportedModels.
- ChannelService.ListAvailable() aggregates channels + active groups;
  filters out group IDs no longer active.
- Admin route GET /api/v1/admin/channels/available returns the full
  DTO (id, status, billing_model_source, restrict_models, groups,
  supported_models).
- User route GET /api/v1/channels/available applies three filters:
  Status==active, visible-group intersection, and platform filter
  on supported_models (prevents cross-platform leak when a channel
  links to both a user-accessible group and an inaccessible one on
  another platform). Response is a plain array (matches the
  /groups/available sibling shape). Field whitelist omits
  billing_model_source, restrict_models, ids, status, sort_order.

Frontend
- New /admin/available-channels and /available-channels views backed
  by a shared AvailableChannelsTable component (admin adds status +
  billing-source columns via slots).
- PricingRow extracted to its own SFC; SupportedModelChip references
  shared billing-mode constants in constants/channel.ts.
- Sidebar: new entry above "渠道管理" for admin; matching entry in
  user nav.
- i18n: zh + en coverage for both namespaces.

Tests
- SupportedModels: wildcard-only pricing skipped, prefix-matches-
  nothing, cross-platform bleed, case-insensitive dedup, empty
  platform mapping.
- ListAvailable: nil groupRepo, inactive-group-ID dropped, stable
  case-insensitive name sort.
- User handler: 401 on unauthenticated, visible-group intersection,
  platform filter on supported_models, JSON whitelist.
- Admin handler: full DTO including default BillingModelSource
  fallback.

Refs: issue #1729

backend/internal/service/channel_available_test.go

@@ -0,0 +1,119 @@
+//go:build unit
+
+package service
+
+import (
+	"context"
+	"testing"
+
+	"github.com/Wei-Shaw/sub2api/internal/pkg/pagination"
+	"github.com/stretchr/testify/require"
+)
+
+// stubGroupRepoForAvailable 是 ListAvailable 测试用的 GroupRepository stub，
+// 仅实现 ListActive；其他方法对本测试无关，返回零值即可。
+type stubGroupRepoForAvailable struct {
+	activeGroups []Group
+}
+
+func (s *stubGroupRepoForAvailable) ListActive(ctx context.Context) ([]Group, error) {
+	return s.activeGroups, nil
+}
+
+func (s *stubGroupRepoForAvailable) Create(ctx context.Context, group *Group) error { return nil }
+func (s *stubGroupRepoForAvailable) GetByID(ctx context.Context, id int64) (*Group, error) {
+	return nil, nil
+}
+func (s *stubGroupRepoForAvailable) GetByIDLite(ctx context.Context, id int64) (*Group, error) {
+	return nil, nil
+}
+func (s *stubGroupRepoForAvailable) Update(ctx context.Context, group *Group) error { return nil }
+func (s *stubGroupRepoForAvailable) Delete(ctx context.Context, id int64) error     { return nil }
+func (s *stubGroupRepoForAvailable) DeleteCascade(ctx context.Context, id int64) ([]int64, error) {
+	return nil, nil
+}
+func (s *stubGroupRepoForAvailable) List(ctx context.Context, params pagination.PaginationParams) ([]Group, *pagination.PaginationResult, error) {
+	return nil, nil, nil
+}
+func (s *stubGroupRepoForAvailable) ListWithFilters(ctx context.Context, params pagination.PaginationParams, platform, status, search string, isExclusive *bool) ([]Group, *pagination.PaginationResult, error) {
+	return nil, nil, nil
+}
+func (s *stubGroupRepoForAvailable) ListActiveByPlatform(ctx context.Context, platform string) ([]Group, error) {
+	return nil, nil
+}
+func (s *stubGroupRepoForAvailable) ExistsByName(ctx context.Context, name string) (bool, error) {
+	return false, nil
+}
+func (s *stubGroupRepoForAvailable) GetAccountCount(ctx context.Context, groupID int64) (int64, int64, error) {
+	return 0, 0, nil
+}
+func (s *stubGroupRepoForAvailable) DeleteAccountGroupsByGroupID(ctx context.Context, groupID int64) (int64, error) {
+	return 0, nil
+}
+func (s *stubGroupRepoForAvailable) GetAccountIDsByGroupIDs(ctx context.Context, groupIDs []int64) ([]int64, error) {
+	return nil, nil
+}
+func (s *stubGroupRepoForAvailable) BindAccountsToGroup(ctx context.Context, groupID int64, accountIDs []int64) error {
+	return nil
+}
+func (s *stubGroupRepoForAvailable) UpdateSortOrders(ctx context.Context, updates []GroupSortOrderUpdate) error {
+	return nil
+}
+
+// newAvailableChannelService 构造一个 ChannelService，channelRepo.ListAll 返回给定 channels，
+// groupRepo 由参数决定（可传 nil 测试 nil 分支）。
+func newAvailableChannelService(channels []Channel, groupRepo GroupRepository) *ChannelService {
+	repo := &mockChannelRepository{
+		listAllFn: func(ctx context.Context) ([]Channel, error) { return channels, nil },
+	}
+	return NewChannelService(repo, groupRepo, nil)
+}
+
+func TestListAvailable_NilGroupRepo_NoGroupsAttached(t *testing.T) {
+	// groupRepo 为 nil 时不应 panic，且每个渠道的 Groups 应为空切片。
+	channels := []Channel{{
+		ID:       1,
+		Name:     "chA",
+		Status:   StatusActive,
+		GroupIDs: []int64{10, 20},
+	}}
+	svc := newAvailableChannelService(channels, nil)
+	out, err := svc.ListAvailable(context.Background())
+	require.NoError(t, err)
+	require.Len(t, out, 1)
+	require.Empty(t, out[0].Groups)
+}
+
+func TestListAvailable_InactiveGroupIDSilentlyDropped(t *testing.T) {
+	// 渠道 GroupIDs 中引用的 group 未出现在 ListActive 结果中（已停用或删除），应被静默丢弃。
+	channels := []Channel{{
+		ID:       1,
+		Name:     "chA",
+		Status:   StatusActive,
+		GroupIDs: []int64{1, 99},
+	}}
+	groupRepo := &stubGroupRepoForAvailable{
+		activeGroups: []Group{{ID: 1, Name: "g1", Platform: "anthropic"}},
+	}
+	svc := newAvailableChannelService(channels, groupRepo)
+	out, err := svc.ListAvailable(context.Background())
+	require.NoError(t, err)
+	require.Len(t, out, 1)
+	require.Len(t, out[0].Groups, 1)
+	require.Equal(t, int64(1), out[0].Groups[0].ID)
+}
+
+func TestListAvailable_SortedByName(t *testing.T) {
+	channels := []Channel{
+		{ID: 1, Name: "beta"},
+		{ID: 2, Name: "Alpha"},
+		{ID: 3, Name: "charlie"},
+	}
+	svc := newAvailableChannelService(channels, nil)
+	out, err := svc.ListAvailable(context.Background())
+	require.NoError(t, err)
+	require.Len(t, out, 3)
+	require.Equal(t, "Alpha", out[0].Name)
+	require.Equal(t, "beta", out[1].Name)
+	require.Equal(t, "charlie", out[2].Name)
+}