feat: concurrency guards, account validation, and reversal side-effects (#247)

* feat: add concurrency guards, account validation, and reversal side-effects to bookkeeping engine

Prevent double-booking via CAS guards on mark-paid and categorize routes (409 on conflict),
make payment GL entries blocking (AP/AR must match GL), validate account resolution in engine,
and auto-sync invoice status on payment reversal. Adds journal_entry.reversed event type.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: address Greptile review — company_id filter, voucher gaps, status restore

- Add missing company_id filter on supplier-invoice CAS update (defense in depth)
- Add voucher_gap_explanations insert on CAS-cancelled entries in both mark-paid
  routes (BFNAR 2013:2 compliance, matching categorize route pattern)
- Fix reversal status restore: check due_date to determine overdue vs sent/approved
  instead of always reverting to sent/approved
- Rename shadowed reversedLines variable to originalLines (P2 clarity)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: derive reversal payment amount from payments table, not GL lines

The reversal GL entry is already a line-by-line mirror per BFL 5 kap 5§.
For the business-level invoice sync, use the payment record amount from
supplier_invoice_payments / invoice_payments instead of inspecting GL
account numbers — works identically for kontantmetod and faktureringsmetod
without needing to know which accounts were used.

Also adds company_id filter on all reversal sync queries (defense in depth).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix: allow reversal of partially_paid customer invoices

Widen the status filter from .eq('status', 'paid') to
.in('status', ['paid', 'partially_paid']) so that reversing a partial
payment GL entry correctly updates the invoice state.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/core/bookkeeping/__tests__/storno-service.test.ts

@@ -163,7 +163,7 @@ describe('correctEntry', () => {
       { data: reversalEntry, error: null },           // 1: insert reversal
       { data: null, error: null },                    // 2: insert reversal lines
       { data: null, error: null },                    // 3: post reversal
-      { data: [], error: null },                      // 4: accounts
+      { data: [{ id: 'acc-5420', account_number: '5420' }, { id: 'acc-1930', account_number: '1930' }], error: null }, // 4: accounts
       { data: null, error: { message: 'DB error' } }, // 5: insert corrected FAILS
       { data: null, error: null },                    // 6: cancelEntry reversal update
       { data: null, error: null },                    // 7: cancelEntry reversal lines delete

lib/core/bookkeeping/storno-service.ts

@@ -174,6 +174,14 @@ export async function correctEntry(
       accountIdMap.set(account.account_number, account.id)
     }
 
+    // Validate all account numbers resolved to IDs
+    const missingAccounts = accountNumbers.filter(num => !accountIdMap.has(num))
+    if (missingAccounts.length > 0) {
+      throw new Error(
+        `Account(s) not found in chart of accounts: ${missingAccounts.join(', ')}`
+      )
+    }
+
     const { data: newEntry, error: correctedError } = await supabase
       .from('journal_entries')
       .insert({