Architect audit P4-K. Add `createdAt`/`updatedAt` TIMESTAMPTZ columns
to 18 tables and flip every model from `timestamps: false` to
`timestamps: true` so Sequelize auto-populates them on every write.

Why
- Auditability: every row carries creation + last-modification time
  without each controller maintaining it by hand.
- Sync clients: third-party integrations get a reliable "what's
  changed since T" boundary for delta-pull workflows.
- Observability: ad-hoc analytics on "new customers per day", etc.,
  trivially work off `createdAt` rather than a row counter.

Coverage
- 18 tables touched: ApiKey, ApiMaster, BillingType, Company,
  Customer, CustomerPayment, InventoryItem, InventoryTransactions,
  Invoice, InvoiceJob, Job, ProductEntry, PurchaseOrderHeaders,
  PurchaseOrderLines, PurchaseOrderVendors, TimeEntry, VersionInfo,
  Worker.
- IdempotencyKey is INTENTIONALLY excluded — it already manages its
  own `ikCreatedAt` / `ikExpiresAt` pair and a parallel
  createdAt/updatedAt would be redundant and confusing.

Backfill
- Existing rows have no recorded history; the migration backfills
  both columns to `now()` via the column DEFAULT. Operators carrying
  legacy Atbash SQL Server timestamps can patch real values
  post-migration via a one-off UPDATE.

Down migration
- Drops the two columns from every table. No FKs reference them,
  so the rollback is straight-forward.

Tests
- New `tests/unit/timestamps.test.js` (36 cases) — every model
  asserts `options.timestamps === true` AND
  `rawAttributes.{createdAt,updatedAt}` exist.
- Full suite: 401 pass / 4 skip (was 365/4).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>