refactor: remove PaymentChannel, reuse upstream Channel with features field

- Delete payment_channels table and PaymentChannel Ent schema
- Add `features` column to upstream channels table (migration 095)
- Add Features field to Channel struct, input types, handler request/response
- Payment user/admin handlers now use ChannelService directly
- Remove Channel CRUD from PaymentConfigService and admin payment routes
- Remove "渠道管理" tab from admin orders page (use /admin/channels)

backend/internal/service/channel.go

@@ -39,6 +39,7 @@ type Channel struct {
 	Status             string
 	BillingModelSource string // "requested", "upstream", or "channel_mapped"
 	RestrictModels     bool   // 是否限制模型（仅允许定价列表中的模型）
+	Features           string // 渠道特性描述（JSON 数组），用于支付页面展示
 	CreatedAt          time.Time
 	UpdatedAt          time.Time