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  filters out group IDs no longer active.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/admin/available_channel_handler.go

@@ -0,0 +1,99 @@
+package admin
+
+import (
+	"github.com/Wei-Shaw/sub2api/internal/pkg/response"
+	"github.com/Wei-Shaw/sub2api/internal/service"
+
+	"github.com/gin-gonic/gin"
+)
+
+// AvailableChannelHandler 处理「可用渠道」聚合视图的管理员接口。
+//
+// 该视图以只读方式聚合渠道基础信息、关联分组与推导出的支持模型列表（无通配符）。
+type AvailableChannelHandler struct {
+	channelService *service.ChannelService
+}
+
+// NewAvailableChannelHandler 创建 AvailableChannelHandler 实例。
+func NewAvailableChannelHandler(channelService *service.ChannelService) *AvailableChannelHandler {
+	return &AvailableChannelHandler{channelService: channelService}
+}
+
+// availableGroupResponse 响应中的分组概要。
+type availableGroupResponse struct {
+	ID       int64  `json:"id"`
+	Name     string `json:"name"`
+	Platform string `json:"platform"`
+}
+
+// supportedModelResponse 响应中的支持模型条目。
+type supportedModelResponse struct {
+	Name     string                       `json:"name"`
+	Platform string                       `json:"platform"`
+	Pricing  *channelModelPricingResponse `json:"pricing"`
+}
+
+// availableChannelResponse 管理员视图完整字段集。
+type availableChannelResponse struct {
+	ID                 int64                    `json:"id"`
+	Name               string                   `json:"name"`
+	Description        string                   `json:"description"`
+	Status             string                   `json:"status"`
+	BillingModelSource string                   `json:"billing_model_source"`
+	RestrictModels     bool                     `json:"restrict_models"`
+	Groups             []availableGroupResponse `json:"groups"`
+	SupportedModels    []supportedModelResponse `json:"supported_models"`
+}
+
+// AvailableChannelToAdminResponse 将 service 层的 AvailableChannel 转为管理员 DTO。
+// 导出供同 package 的复用；也用于构造测试 fixture。
+func AvailableChannelToAdminResponse(ch service.AvailableChannel) availableChannelResponse {
+	groups := make([]availableGroupResponse, 0, len(ch.Groups))
+	for _, g := range ch.Groups {
+		groups = append(groups, availableGroupResponse{ID: g.ID, Name: g.Name, Platform: g.Platform})
+	}
+	models := make([]supportedModelResponse, 0, len(ch.SupportedModels))
+	for i := range ch.SupportedModels {
+		m := ch.SupportedModels[i]
+		var pricing *channelModelPricingResponse
+		if m.Pricing != nil {
+			p := pricingToResponse(m.Pricing)
+			pricing = &p
+		}
+		models = append(models, supportedModelResponse{
+			Name:     m.Name,
+			Platform: m.Platform,
+			Pricing:  pricing,
+		})
+	}
+	billingSource := ch.BillingModelSource
+	if billingSource == "" {
+		billingSource = service.BillingModelSourceChannelMapped
+	}
+	return availableChannelResponse{
+		ID:                 ch.ID,
+		Name:               ch.Name,
+		Description:        ch.Description,
+		Status:             ch.Status,
+		BillingModelSource: billingSource,
+		RestrictModels:     ch.RestrictModels,
+		Groups:             groups,
+		SupportedModels:    models,
+	}
+}
+
+// List 列出所有可用渠道（管理员视图）。
+// GET /api/v1/admin/channels/available
+func (h *AvailableChannelHandler) List(c *gin.Context) {
+	channels, err := h.channelService.ListAvailable(c.Request.Context())
+	if err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+
+	out := make([]availableChannelResponse, 0, len(channels))
+	for _, ch := range channels {
+		out = append(out, AvailableChannelToAdminResponse(ch))
+	}
+	response.Success(c, gin.H{"items": out})
+}