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/schema.prisma

@@ -130,7 +130,8 @@ model Customer {
   transactions Transaction[]
 
   statementDocuments StatementDocument[]
-  policyOcrDocuments PolicyOcrDocument[] @relation("PolicyOcrDocumentCustomer")
+  statementSplits    StatementDocumentSplit[]
+  policyOcrDocuments PolicyOcrDocument[]      @relation("PolicyOcrDocumentCustomer")
 
   emailNotificationLogs EmailNotificationLog[]
   accountStatusHistory  AccountStatusHistory[]
@@ -730,6 +731,26 @@ model StatementBatch {
 }
 
 /// One statement — one customer, one period — after splitting the batch.
+/// One customer's share of a split receipt — a water bill two neighbours
+/// share, an HOA fee spread across a building. Free amounts, no stored
+/// percentages: the office splits unevenly as often as not.
+model StatementDocumentSplit {
+  id         String            @id @default(uuid())
+  documentId String
+  document   StatementDocument @relation(fields: [documentId], references: [id], onDelete: Cascade)
+  /// 1-based; also the `#n` in the posted row's captureRef.
+  sequence   Int
+  customerId String
+  customer   Customer          @relation(fields: [customerId], references: [id])
+  amount     Decimal           @db.Decimal(12, 2)
+
+  postedTransactionId String?      @unique
+  postedTransaction   Transaction? @relation("StatementDocumentSplitTransaction", fields: [postedTransactionId], references: [id])
+
+  @@unique([documentId, sequence])
+  @@map("statement_document_splits")
+}
+
 model StatementDocument {
   id         String                  @id @default(uuid())
   batchId    String
@@ -775,6 +796,15 @@ model StatementDocument {
   postedTransactionId String?      @unique
   postedTransaction   Transaction? @relation(fields: [postedTransactionId], references: [id])
 
+  /// Sin fondos for this receipt alone: posted as outstanding even when the
+  /// rest of the check is funded.
+  outstanding Boolean @default(false)
+
+  /// When set, the receipt is paid for by several customers and posts one
+  /// charge per part instead of one to `matchedCustomer`. The parts always sum
+  /// to `extractedAmount`.
+  splits StatementDocumentSplit[]
+
   createdAt DateTime @default(now())
 
   @@unique([batchId, pageNumber])
@@ -853,7 +883,8 @@ model Transaction {
 
   /// Set only on OCR-posted rows — the statement page this came from.
   statementDocument StatementDocument?
-  policyOcrDocument PolicyOcrDocument? @relation("PolicyOcrDocumentTransaction")
+  statementSplit    StatementDocumentSplit? @relation("StatementDocumentSplitTransaction")
+  policyOcrDocument PolicyOcrDocument?      @relation("PolicyOcrDocumentTransaction")
 
   @@unique([legacySourceDb, legacySourceTable, legacyId])
   @@index([customerId, transactionDate])