docs: add AD-03 cutover and rollback procedures

[IsurangiGuniyangodage]

Why

AD-03 (MongoDB → PostgreSQL migration) needs a documented cutover and rollback plan before backend refactor work (AD-BE-T4–T6) proceeds. Schema design and auth refactor (AD-BE-T3) status is tracked separately on the board; at the time this PR was opened, migration/reconciliation scripts (AD-BE-T2) had not yet been merged into main.

What changed

• Added Documentation/AD-03-cutover-rollback.md
• Documents planned cutover sequence, rollback triggers, and rollback steps
• Flagged that AD-BE-T2 scripts were not yet present in main at time of writing — doc was marked draft pending validation

Notes

Update: AD-BE-T2 migration/reconciliation scripts now exist and are built out on Adriana's feature/ad-be-t2-migration-reconciliation branch, not yet merged to main. This doc's cutover/rollback plan should be revisited against her actual FINALISED_POSTGRESQL_SCHEMA.md architecture once that branch merges, since the target schema shape (DE-owned silver tables vs. App-owned app schema) is more specific than what was assumed when this doc was originally written.

[IsurangiGuniyangodage]

@callmesoffie1811 , could you review this when you get a chance?

[callmesoffie1811]

Thanks for documenting the AD-03 cutover and rollback process. The scope, planned sequence, rollback triggers and known limitations are clearly documented.

I don't see any showcase-blocking issues, and the PR is mergeable. ✅

I'm happy for this to proceed mentor approval process. The document can be revisited once AD-BE-T2 and the final PostgreSQL schema are merged/confirmed.