[#1] FIX: Flyway 마이그레이션 SQL 수정

[Hwan0518]

Issue ✨

• [Chore] Init #1

버그 해결 💊

• Flyway 마이그레이션 SQL 수정

[Copilot]

Pull request overview

This PR updates a Flyway migration to use simpler, database-native syntax when adding the net_payload_weight column to the trade_item table. The goal is to fix a migration bug while keeping the script idempotent and compatible with the project's MariaDB 10.6 environment.

Changes:

• Replace the previous information_schema + dynamic SQL approach with a direct ALTER TABLE ... ADD COLUMN IF NOT EXISTS statement for net_payload_weight.
• Retain the existing backfill and NOT NULL constraint steps to ensure all rows get a valid net_payload_weight value.

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