fix: validate account mappings and handle reverse charge in categorization

Add validation for non-empty debit/credit accounts before creating journal
entries. Generate fiktiv moms lines (2645/2614) for EU reverse charge expenses.
Change default unmapped expense account from 6900 to 6991. Add tests for
reverse charge handling and exhaustive category mapping coverage.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/api/transactions/[id]/categorize/__tests__/route.test.ts

@@ -228,6 +228,33 @@ describe('POST /api/transactions/[id]/categorize', () => {
     expect(body.error).toBe('Failed to update transaction')
   })
 
+  it('returns 400 when mapping result has empty debit_account', async () => {
+    const tx = makeTransaction({
+      id: 'tx-1',
+      amount: -500,
+      journal_entry_id: null,
+    })
+
+    enqueue({ data: tx, error: null })
+    enqueue({ data: { entity_type: 'enskild_firma', fiscal_year_start_month: 1 }, error: null })
+
+    mockBuildMappingResultFromCategory.mockReturnValue({
+      ...defaultMappingResult,
+      debit_account: '',
+    })
+
+    const request = createMockRequest('/api/transactions/tx-1/categorize', {
+      method: 'POST',
+      body: { is_business: true, category: 'expense_software' },
+    })
+    const response = await POST(request, createMockRouteParams({ id: 'tx-1' }))
+    const { status, body } = await parseJsonResponse<{ error: string }>(response)
+
+    expect(status).toBe(400)
+    expect(body.error).toBe('Invalid account mapping: debit and credit accounts are required')
+    expect(mockCreateTransactionJournalEntry).not.toHaveBeenCalled()
+  })
+
   it('categorizes as private when is_business is false', async () => {
     const tx = makeTransaction({
       id: 'tx-1',

app/api/transactions/[id]/categorize/route.ts

@@ -199,6 +199,14 @@ export async function POST(
     }
   }
 
+  // Validate that both accounts are present before proceeding
+  if (!mappingResult.debit_account || !mappingResult.credit_account) {
+    return NextResponse.json(
+      { error: 'Invalid account mapping: debit and credit accounts are required' },
+      { status: 400 }
+    )
+  }
+
   // Ensure fiscal period exists for the transaction date
   await ensureFiscalPeriod(supabase, user.id, transaction.date, fiscalYearStartMonth)