feat(payments): payment-batch API surface (betalfil 2/3) (#1504)

Preview/create/list/get/file/cancel routes under
/api/supplier-invoices/payment-batches, SI_BATCH_* structured errors,
and a fail-closed batch-membership pre-check in the supplier invoice
DELETE route (the FK RESTRICT is the backstop). File downloads stamp
file_generated_at + download_count but regenerate byte-identically.

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

lib/api/schemas.ts

@@ -1081,6 +1081,39 @@ export const UpdateSupplierInvoiceSchema = z.object({
   notes: z.string().optional(),
 })
 
+// ============================================================
+// Supplier payment batch (betalfil) schemas
+// ============================================================
+
+// v1 gates the API to pain001; the DB CHECK also allows 'bg_lb' so a future
+// format lands without a migration.
+const supplierPaymentBatchFormat = z.enum(['pain001'])
+
+export const PreviewSupplierPaymentBatchSchema = z.object({
+  format: supplierPaymentBatchFormat,
+  ids: z.array(z.string().uuid()).min(1).max(100),
+})
+
+export const SupplierPaymentBatchItemInputSchema = z.object({
+  supplier_invoice_id: z.string().uuid(),
+  // Defaults to the invoice's remaining amount.
+  amount: z.number().positive().optional(),
+  // Defaults to max(due_date, today); past dates are normalized to today.
+  payment_date: isoDate.optional(),
+})
+
+export const CreateSupplierPaymentBatchSchema = z.object({
+  format: supplierPaymentBatchFormat,
+  items: z.array(SupplierPaymentBatchItemInputSchema).min(1).max(100),
+  confirm_already_batched: z.boolean().optional(),
+})
+
+export const SupplierPaymentBatchListQuerySchema = z.object({
+  status: z.enum(['created', 'cancelled', 'all']).default('all'),
+  limit: z.coerce.number().int().min(1).max(100).default(50),
+  offset: z.coerce.number().int().min(0).default(0),
+})
+
 // ============================================================
 // Journal entry schemas
 // ============================================================