fix(crm): thread trx through sequence-claim sites to unblock SQLite

Reviewer feedback on #555: nextQuoteNumber inside createQuote's
db.transaction was called without passing the outer trx, so
claimNextSequence opened its own connection — Postgres tolerated this
via the pool, SQLite (1-connection default) deadlocked on every quote
creation.
Audited the same pattern across invoiceService + contractService and
found five more matching call sites:
  - createInvoice (single-row path after installment auto-route)
  - spawnInstallmentInvoices (per-sibling claim inside the loop)
  - createStorno
  - createContract
  - createFromQuote
All now thread trx through to nextXxxNumber → claimNextSequence so
the claim joins the caller's transaction on both engines.
convertToInvoiceOnly's Path B (standalone-contract) is the lone
remaining nextInvoiceNumber() call without trx — that path isn't
wrapped in a transaction at all (separate concern: sequence-number
leak on insert failure, tracked separately).

backend/src/services/contractService.js

@@ -765,7 +765,9 @@ async function createContract(payload, adminId) {
   const hasEventCols = await hasColumnCached('contracts', 'event_name');
 
   return await db.transaction(async (trx) => {
-    const contractNumber = await nextContractNumber();
+    // Pass trx so the sequence claim joins our outer transaction —
+    // SQLite deadlocks otherwise (1-connection default).
+    const contractNumber = await nextContractNumber(trx);
     const row = {
       contract_number: contractNumber,
       customer_account_id: payload.customerAccountId,
@@ -1594,7 +1596,9 @@ async function createFromQuote(quoteId, adminId) {
   const hasContractEventCols = await hasColumnCached('contracts', 'event_name');
 
   return await db.transaction(async (trx) => {
-    const contractNumber = await nextContractNumber();
+    // Pass trx so the sequence claim joins our outer transaction —
+    // SQLite deadlocks otherwise (1-connection default).
+    const contractNumber = await nextContractNumber(trx);
     const contractRow = {
       contract_number: contractNumber,
       customer_account_id: quote.customer_account_id,