fix: implement down in add-loan-events-missing-indexes to drop indexes

[SharifIbrahimDev]

Closes #1193

What does this PR do?

This PR implements the missing down() function in the 1788000000018_add-loan-events-missing-indexes.js migration file, which previously leaked four database indexes on rollback.

Description

• Clean Rollbacks: Replaced the empty down = () => {} with explicit DROP INDEX IF EXISTS statements for all four indexes (idx_loan_events_borrower, idx_loan_events_event_type, idx_loan_events_loan_id_event_type, and idx_loan_events_created_at) created in the up() function.
• Idempotency: Ensures that rolling back and re-applying migrations works flawlessly without index collision errors.

[ogazboiz]

reviewed the content and it's good: the down() drops exactly the four indexes the up() creates (created_at, loan_id_event_type, event_type, borrower) with IF EXISTS, and i verified the full up -> down-all -> up-again cycle applies cleanly on postgres against the current migration set. two things before it can merge:

1. this branch is ~40 commits behind main (it predates the commonjs->esm migration conversion), which is why ci is red, it's stale-base, not your change. please rebase: git fetch origin && git rebase origin/main && git push --force-with-lease.
2. drop the stray pr_body_1193.md from the repo root (that belongs in the pr description, not the tree).

non-blocking: idx_loan_events_loan_id_event_type is also created by the earlier 1777 composite-indexes migration, so a partial rollback of just 1788 would drop an index 1777 also owns, the IF EXISTS guards keep it safe but it's worth a one-line comment. rebase + drop the md and i'll merge.

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