feat(db): add migration rollback support for all migrations

[favvy0217]

Summary

Resolves #1796

Adds -- @down rollback sections to every SQL migration in db/migrations/ and backend/migrations/ so that node scripts/db-migrate-run.js down can revert any applied migration.

Changes

• Add inline -- @down sections to all 10 db/migrations/*.sql files
• Add inline -- @down sections to both backend/migrations/*.sql files
• Create scripts/__tests__/migration-rollback-parity.test.js to verify every migration has a non-trivial rollback path
• Create db/MIGRATION_ROLLBACK.md documenting the rollback workflow, commands, and safety guarantees

Acceptance Criteria

• Rollback tested — dry-run confirms all 12 migrations have down paths
• Documented — db/MIGRATION_ROLLBACK.md covers commands and safety
• Safe — each down section reverses its up counterpart; ledger consistency is maintained by the transactional runner

Verification

node scripts/db-migrate-dryrun.js --migrations-dir db/migrations
# All migrations show: Rollback path: ✓ Present

node scripts/db-migrate-dryrun.js --migrations-dir backend/migrations
# All migrations show: Rollback path: ✓ Present

[drips-wave]

@favvy0217 Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits