feat(ledger,bank): append + void write API, voided excluded from totals (plan phase 5 API)

Transactions and the bank register become append-only with a void
(reversal) action — never edited or hard-deleted. This is the API half of
phase 5; the capture/void web UI is the remaining piece.

Schema:
- Transaction and BankTransaction gain voidedAt + voidedById. A non-null
  voidedAt reverses the row. Pushed to dev.

Correctness (the high-stakes part):
- Every aggregate excludes voided rows: billing movements totals, the raw
  balances SQL, stats (groupBy + the sides/crossLine raw subqueries +
  first/last), facets (types/sources/years); the statement's running
  balance freezes on a voided row and its per-currency/per-domain/per-type
  summaries skip them; customers.detail and property owner-ledger groupBy;
  and every bank total (totalsFor, stats counts/bounds, facets + summary
  raw SQL). List views still return voided rows with a `voided` flag so
  the UI can strike them through.
- Bank's legacy zero-amount "void" cheques are unchanged and distinct from
  app voids (voidedAt).

API:
- POST /billing + POST /billing/:id/void (ledger:create / ledger:void);
  POST /bank + POST /bank/:id/void (bank:create / bank:void). Create needs
  STAFF+, void needs MANAGER+. Double-void -> 400, unknown id -> 404,
  bad date -> 400. Mutations audited. DTOs added.

Verified against dev end-to-end: a -500 MXN charge moved a customer
balance 31082.08 -> 30582.08, and voiding it returned it to 31082.08 to
the cent; a +1234.56 bank ingreso moved net 899375.77 -> 900610.33 and
voiding returned it to 899375.77. VIEWER create/void both 403,
double-void 400. API compiles clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/api/src/customers/customers.service.ts

@@ -132,7 +132,8 @@ export class CustomersService {
     // business lines" payoff), computed in the DB rather than in JS.
     const summary = await this.prisma.transaction.groupBy({
       by: ["domain", "currency"],
-      where: { customerId: id },
+      // Exclude voided rows so the per-domain balance matches the statement.
+      where: { customerId: id, voidedAt: null },
       _sum: { amount: true },
       _count: { _all: true },
     });