P5-3: Create Flyway migration script

[tcheeric]

Description:
Add Flyway migration for new GatewayPayment columns and constraints.

Changes required:

• Create Flyway migration script for new columns and constraint.
• Verify migration ordering and naming.

Acceptance criteria:

• Migration script applies cleanly on a fresh database.
• Existing installations can migrate without errors.

Estimated hours: TBD
Priority: Medium
Branch name: refactor/phase-5-task-3
Dependencies: P5-2