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>

packages/database/prisma/schema.prisma

@@ -409,6 +409,11 @@ model Transaction {
   checkNumber       String?
   message           String? @db.Text
   outstanding       Boolean           @default(false)
+  // Append + void: booked rows are never edited or hard-deleted. A non-null
+  // voidedAt reverses the movement — it MUST be excluded from every balance
+  // and total (SUM/count) so a voided amount stops affecting the books.
+  voidedAt          DateTime?
+  voidedById        String?
   legacySourceDb    String?
   legacySourceTable String?
   legacyId          String?
@@ -457,6 +462,9 @@ model BankTransaction {
   transferred       Boolean               @default(false)
   notes             String? @db.Text
   amountInWords     String?
+  // Append + void (see Transaction.voidedAt): excluded from income/expense/net.
+  voidedAt          DateTime?
+  voidedById        String?
   legacySourceTable String?
   legacyId          String?