Add safe decimal migration tooling

[kimanicode]

Summary

Closes #238.

Adds safe Float-to-Decimal migration tooling for money-like database columns. The script supports dry-run row counts, NaN/Infinity preflight rejection, gzipped JSONL backups, pg_dump --data-only safety backups, single-transaction conversion to NUMERIC, sentinel-based idempotency, and rollback restore from backup archives.

Also adds a database migration runbook and an npm script entry:

npm run decimal:migrate

## Testing 
npx tsc -p tsconfig.json --noEmit --pretty false
npx eslint scripts/decimal-migrate.ts
npx prettier --check scripts/decimal-migrate.ts ../../docs/db/decimal-migration.md package.json
npm run decimal:migrate -- --help

I did not run the migration against a live PostgreSQL database because no DATABASE_URL was provided.

Screenshots

Not applicable.

Checklist

• Tests added or updated for new/changed behavior
• Existing tests pass
• No secrets, keys, or seed phrases committed
• Follows the coding conventions in the relevant service README
• PR is focused on a single concern

[kilodesodiq-arch]

LGTM