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  filters out group IDs no longer active.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/handler/available_channel_handler_test.go

@@ -0,0 +1,121 @@
+//go:build unit
+
+package handler
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/Wei-Shaw/sub2api/internal/service"
+
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/require"
+)
+
+func TestUserAvailableChannel_Unauthenticated401(t *testing.T) {
+	// 没有 AuthSubject 注入时，handler 应返回 401 且不触达 service 依赖。
+	gin.SetMode(gin.TestMode)
+	h := &AvailableChannelHandler{} // nil services — 401 路径不会调用它们
+	w := httptest.NewRecorder()
+	c, _ := gin.CreateTestContext(w)
+	c.Request = httptest.NewRequest(http.MethodGet, "/api/v1/channels/available", nil)
+
+	h.List(c)
+
+	require.Equal(t, http.StatusUnauthorized, w.Code)
+}
+
+func TestFilterUserVisibleGroups_IntersectionOnly(t *testing.T) {
+	// 渠道挂在 {g1, g2, g3}，用户只允许 {g1, g3} —— 响应必须仅含 g1/g3。
+	groups := []service.AvailableGroupRef{
+		{ID: 1, Name: "g1", Platform: "anthropic"},
+		{ID: 2, Name: "g2", Platform: "anthropic"},
+		{ID: 3, Name: "g3", Platform: "openai"},
+	}
+	allowed := map[int64]struct{}{1: {}, 3: {}}
+
+	visible := filterUserVisibleGroups(groups, allowed)
+	require.Len(t, visible, 2)
+	ids := []int64{visible[0].ID, visible[1].ID}
+	require.ElementsMatch(t, []int64{1, 3}, ids)
+}
+
+func TestCollectGroupPlatforms_DerivesAllowedSet(t *testing.T) {
+	groups := []userAvailableGroup{
+		{ID: 1, Platform: "anthropic"},
+		{ID: 2, Platform: "openai"},
+		{ID: 3, Platform: "anthropic"}, // 去重
+		{ID: 4, Platform: ""},          // 空平台忽略
+	}
+	got := collectGroupPlatforms(groups)
+	require.Len(t, got, 2)
+	_, hasAnt := got["anthropic"]
+	_, hasOA := got["openai"]
+	require.True(t, hasAnt)
+	require.True(t, hasOA)
+}
+
+func TestToUserSupportedModels_FiltersByAllowedPlatforms(t *testing.T) {
+	// 用户可访问分组只覆盖 anthropic；anthropic 平台的模型保留，openai 模型被剔除。
+	src := []service.SupportedModel{
+		{Name: "claude-sonnet-4-6", Platform: "anthropic", Pricing: nil},
+		{Name: "gpt-4o", Platform: "openai", Pricing: nil},
+	}
+	allowed := map[string]struct{}{"anthropic": {}}
+	out := toUserSupportedModels(src, allowed)
+	require.Len(t, out, 1)
+	require.Equal(t, "claude-sonnet-4-6", out[0].Name)
+}
+
+func TestToUserSupportedModels_NilAllowedPlatformsKeepsAll(t *testing.T) {
+	// 显式传 nil allowedPlatforms 表示不做过滤。
+	src := []service.SupportedModel{
+		{Name: "a", Platform: "anthropic"},
+		{Name: "b", Platform: "openai"},
+	}
+	require.Len(t, toUserSupportedModels(src, nil), 2)
+}
+
+func TestUserAvailableChannel_FieldWhitelist(t *testing.T) {
+	// 通过序列化 userAvailableChannel 结构体验证响应形状：
+	// 只有 name / description / groups / supported_models；不含管理端字段。
+	row := userAvailableChannel{
+		Name:            "ch",
+		Description:     "d",
+		Groups:          []userAvailableGroup{{ID: 1, Name: "g1", Platform: "anthropic"}},
+		SupportedModels: []userSupportedModel{},
+	}
+	raw, err := json.Marshal(row)
+	require.NoError(t, err)
+	var decoded map[string]any
+	require.NoError(t, json.Unmarshal(raw, &decoded))
+
+	for _, key := range []string{"id", "status", "billing_model_source", "restrict_models"} {
+		_, exists := decoded[key]
+		require.Falsef(t, exists, "user DTO must not expose %q", key)
+	}
+	for _, key := range []string{"name", "description", "groups", "supported_models"} {
+		_, exists := decoded[key]
+		require.Truef(t, exists, "user DTO must expose %q", key)
+	}
+
+	// pricing interval 白名单：不应暴露 id / sort_order。
+	pricing := toUserPricing(&service.ChannelModelPricing{
+		BillingMode: service.BillingModeToken,
+		Intervals: []service.PricingInterval{
+			{ID: 7, MinTokens: 0, MaxTokens: nil, SortOrder: 3},
+		},
+	})
+	require.NotNil(t, pricing)
+	require.Len(t, pricing.Intervals, 1)
+	rawIv, err := json.Marshal(pricing.Intervals[0])
+	require.NoError(t, err)
+	var ivDecoded map[string]any
+	require.NoError(t, json.Unmarshal(rawIv, &ivDecoded))
+	for _, key := range []string{"id", "pricing_id", "sort_order"} {
+		_, exists := ivDecoded[key]
+		require.Falsef(t, exists, "user pricing interval must not expose %q", key)
+	}
+}