feat(db): additive prep migration for the henza data import

Purely additive; nothing dropped, renamed or retyped. Owner rulings of
2026-10-06/07, recorded in docs/DATA-MIGRATION-MAP.md.

- Customer.nationalCode is now optional. All 1,595 live legacy customers
  have none, and fabricating one is a data violation. Still @unique:
  NULLs do not collide in Postgres, so the import must write NULL, not ''.
- Customer.birthDate added, empty. Must not be backfilled until the
  Jalali-vs-Gregorian check (precheck-3) is in.
- VerificationStep.REJECTED plus verificationRejectionReason/RejectedAt.
  Legacy has a third "rejected" state that the enum could not represent.
- CustomerFavorite for legacy `fav` (251 rows), which had no home.

accounting-sync customerPayload() now takes `nationalCode: string | null`.
validNationalCode() already handled null, so Tahesab/Zarrin receive
exactly what they did before. Caught by tsc after the column change.

Not touched: accounting* columns, ProductVariant.imageId, the
Accounting* tables, SMS (PROTECTED.md §B, §G).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/api/src/modules/accounting-sync/accounting-sync.service.ts

@@ -230,7 +230,11 @@ export class AccountingSyncService {
   }
 
   private customerPayload(
-    c: { firstName: string | null; lastName: string | null; fullName: string; mobile: string; phone: string | null; address: string | null; nationalCode: string },
+    // nationalCode is nullable since 2026-10-07 (no legacy customer has one —
+    // docs/DATA-MIGRATION-MAP.md §10.4). validNationalCode() already takes
+    // `string | null` and maps empty/invalid to null, so the providers below
+    // keep receiving exactly what they received before.
+    c: { firstName: string | null; lastName: string | null; fullName: string; mobile: string; phone: string | null; address: string | null; nationalCode: string | null },
     group: ExternalGroup | null,
   ): CustomerPayload {
     const parts = c.firstName || c.lastName ? { firstName: c.firstName ?? '', lastName: c.lastName ?? '' } : splitName(c.fullName);