fix(trading): 补充 p2p_transfers 表遗漏的 fee 列

schema 中定义了 fee 字段但 0002 migration 遗漏，导致
prisma.p2pTransfer.create() 报错 "column fee does not exist"

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/services/trading-service/prisma/migrations/0005_add_fee_to_p2p_transfers/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable: 补充遗漏的 fee 列
+ALTER TABLE "p2p_transfers" ADD COLUMN "fee" DECIMAL(30,8) NOT NULL DEFAULT 0;