Add standalone CLI tool for database migration

[aofei]

Description

With the database migration system implemented in #2001, we now have automated migrations that run when the spx-backend starts. However, there are operational scenarios where a standalone migration tool would be valuable.

Currently, migration operations are tightly coupled with application startup:

• Migrations only run when config.DatabaseConfig.AutoMigrate is enabled
• No way to handle dirty state without direct database manipulation
• Cannot execute migrations independently of application deployment

Possible solution

Create a standalone CLI tool spx-backend-migrate that provides:

• status: Show current migration version and pending migrations
• up [N]: Apply N pending migrations (default: all)
• down [N]: Rollback N migrations (emergency use only)
• goto <version>: Migrate to specific version (up or down as needed)
• force <version>: Force set version to resolve dirty state
• validate: Verify migration file integrity, up/down symmetry, and schema consistency with GORM models

[!NOTE] Following golang-migrate conventions, [N] represents the count of migrations to apply/rollback, not version numbers. Use goto <version> to migrate to a specific version.