feat(recibos): split a receipt, sin fondos per receipt, keep a batch open

Three asks from the 2026-10-08 meeting, all on the OCR receipt queue.

- Split: one receipt paid by several customers (411 + 523 sharing water
  3014057) posts one charge per part. statement_document_splits holds free
  amounts that must sum to the receipt in cents, checked on save and again
  at confirm; "partes iguales" fills them in. Each part's row carries
  captureRef <docId>#<n>, so the double-post guard still holds.
- Sin fondos per receipt: StatementDocument.outstanding, toggled in review.
  At confirm a line is outstanding if its receipt or the whole check is.
- Open batch: POST /statements/batches/:id/files appends scans, numbered
  after the existing pages; a COMPLETED batch reopens. The confirm card
  prefills the check the batch was last posted against.

Verified on dev against a real batch: 1,892.00 split 946/946 across two
customers, a sibling receipt posted outstanding, second confirm refused.

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

packages/database/prisma/migrations/20261013120000_statement_split_outstanding/migration.sql

@@ -0,0 +1,30 @@
+-- Two asks from the 2026-10-08 meeting, both on the OCR receipt queue:
+--  * sin fondos per receipt, not only per check (`outstanding`);
+--  * one receipt paid by several customers (411 + 523 sharing a water bill),
+--    posted as one charge per part.
+-- AlterTable
+ALTER TABLE `statement_documents` ADD COLUMN `outstanding` BOOLEAN NOT NULL DEFAULT false;
+
+-- CreateTable
+CREATE TABLE `statement_document_splits` (
+    `id` VARCHAR(191) NOT NULL,
+    `documentId` VARCHAR(191) NOT NULL,
+    `sequence` INTEGER NOT NULL,
+    `customerId` VARCHAR(191) NOT NULL,
+    `amount` DECIMAL(12, 2) NOT NULL,
+    `postedTransactionId` VARCHAR(191) NULL,
+
+    UNIQUE INDEX `statement_document_splits_postedTransactionId_key`(`postedTransactionId`),
+    UNIQUE INDEX `statement_document_splits_documentId_sequence_key`(`documentId`, `sequence`),
+    PRIMARY KEY (`id`)
+) DEFAULT CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci;
+
+-- AddForeignKey
+ALTER TABLE `statement_document_splits` ADD CONSTRAINT `statement_document_splits_documentId_fkey` FOREIGN KEY (`documentId`) REFERENCES `statement_documents`(`id`) ON DELETE CASCADE ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE `statement_document_splits` ADD CONSTRAINT `statement_document_splits_customerId_fkey` FOREIGN KEY (`customerId`) REFERENCES `customers`(`id`) ON DELETE RESTRICT ON UPDATE CASCADE;
+
+-- AddForeignKey
+ALTER TABLE `statement_document_splits` ADD CONSTRAINT `statement_document_splits_postedTransactionId_fkey` FOREIGN KEY (`postedTransactionId`) REFERENCES `transactions`(`id`) ON DELETE SET NULL ON UPDATE CASCADE;
+