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/migrations/095_channel_features.sql

@@ -0,0 +1,2 @@
+ALTER TABLE channels ADD COLUMN IF NOT EXISTS features TEXT NOT NULL DEFAULT '';
+COMMENT ON COLUMN channels.features IS '渠道特性描述，JSON 数组格式，用于支付页面展示';