README: app, data model, setup and how migrations are gated

[dipakkr]

Summary

Rewrites the README so a new engineer (or a judge) understands ledgerly in two minutes: what the app shows, how the data is shaped, how to run it, and how schema changes are gated.

What changed

• Screenshots: the live dashboard, and a ledgerly migration PR paused at the approval step in TrueForge.
• What's in the app: each dashboard element and the exact query file behind it; live refresh with red deltas when numbers change underneath you.
• Data model: tables, row counts and relationships, including the ON DELETE CASCADE from refunds to payments.
• Known data quirks: 18 double charges (11 refunded), 37 payments of deleted customers, 64 legacy POS payments without a merchant.
• Run it locally: .env, npm run reset, npm start, a Docker one-liner for Postgres, and what deploy:unsafe demonstrates.
• Migrations and CI: the required Migration Rehearsal check, protected tables, PII masking, and the one-command onboarding.

How to test

Docs only. The required check passes with "No migration changes".