fix: resolve migration timestamp collisions

[MerlinTheWhiz]

Closes #22

Summary

Problem

Four pairs of migration files shared identical timestamp prefixes, making their apply order non-deterministic:

Collision	Files
1777000000007	loan-events-composite-indexes, unique-loan-status-events
1778000000008	quarantine-events, transaction-submissions
1786000000016	ensure-loan-events-loan-id-index, webhook-max-attempts
1788000000018	add-loan-events-missing-indexes, unified-contract-events

The 1788000000018 pair was most critical: add-loan-events-missing-indexes creates indexes on loan_events while unified-contract-events renames that table to contract_events. If they ran in the wrong order, the index creation would fail.

Changes

Renamed one file per colliding pair to a unique, strictly-increasing timestamp:

Before	After
1777000000007_unique-loan-status-events.js	1777000000008_unique-loan-status-events.js
1778000000008_transaction-submissions.js	1778000000009_transaction-submissions.js
1786000000016_webhook-max-attempts.js	1786000000017_webhook-max-attempts.js
1788000000018_unified-contract-events.js	1788000000019_unified-contract-events.js

All 27 timestamps are now unique and strictly increasing. The critical pair is correctly ordered — add-loan-events-missing-indexes (1788000000018) runs before unified-contract-events (1788000000019).

README — updated the core migrations table, added a migration naming convention section to prevent future collisions, and documented the exact SQL to fix pgmigrations on existing databases after the rename.

Verification

• All timestamps unique and strictly increasing
• Existing DB + renamed files: node-pg-migrate correctly blocks with order-check error
• Running the provided SQL to sync pgmigrations then allows migrate:up to succeed
• Documented convention to prevent future timestamp collisions