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/handler/payment_handler.go

@@ -7,6 +7,7 @@ import (
 	"github.com/Wei-Shaw/sub2api/internal/pkg/pagination"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/response"
 	middleware2 "github.com/Wei-Shaw/sub2api/internal/server/middleware"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/pagination"
 	"github.com/Wei-Shaw/sub2api/internal/service"
 
 	"github.com/gin-gonic/gin"