feat: Enhance journal entry and transaction handling with posting keys

- Added posting key support to SKR03 and SKR04 journal entries and transactions to ensure DATEV compliance.
- Implemented validation for posting keys in journal entries, ensuring all lines have a posting key and that they are consistent across the entry.
- Introduced automatic account checks to prevent posting to accounts that cannot be directly posted to (e.g., 1400, 1600).
- Updated account validation to include checks for debtor and creditor ranges.
- Enhanced invoice booking logic to include appropriate posting keys based on VAT rates and scenarios.
- Created a new module for posting key definitions and validation rules, including functions for validating posting keys and suggesting appropriate keys based on transaction parameters.
- Updated tests to cover new posting key functionality and ensure compliance with accounting rules.

test/test.transactions.ts

@@ -29,8 +29,8 @@ tap.test('should enforce double-entry bookkeeping rules', async () => {
       description: 'Unbalanced entry',
       reference: 'TEST-001',
       lines: [
-        { accountNumber: '1000', debit: 100 },
-        { accountNumber: '4000', credit: 50 }, // Unbalanced!
+        { accountNumber: '1000', debit: 100, postingKey: 40 },
+        { accountNumber: '4000', credit: 50, postingKey: 40 }, // Unbalanced!
       ],
       skrType: 'SKR03',
     });
@@ -99,10 +99,10 @@ tap.test(
       description: 'Complex distribution',
       reference: 'COMPLEX-001',
       lines: [
-        { accountNumber: '5000', debit: 500, description: 'Materials' },
-        { accountNumber: '6000', debit: 300, description: 'Wages' },
-        { accountNumber: '7100', debit: 200, description: 'Rent' },
-        { accountNumber: '1200', credit: 1000, description: 'Bank payment' },
+        { accountNumber: '5000', debit: 500, description: 'Materials', postingKey: 40 },
+        { accountNumber: '6000', debit: 300, description: 'Wages', postingKey: 40 },
+        { accountNumber: '7100', debit: 200, description: 'Rent', postingKey: 40 },
+        { accountNumber: '1200', credit: 1000, description: 'Bank payment', postingKey: 40 },
       ],
       skrType: 'SKR03',
     });
@@ -220,10 +220,19 @@ tap.test('should handle batch transaction posting', async () => {
 });
 
 tap.test('should handle transaction with VAT', async () => {
+  // Create creditor account for supplier
+  await api.createAccount({
+    accountNumber: '70001',
+    accountName: 'Lieferant Test GmbH',
+    accountClass: 7,
+    accountType: 'liability',
+    skrType: 'SKR03',
+  });
+
   const transaction = await api.postTransaction({
     date: new Date(),
     debitAccount: '5400', // Goods with 19% VAT
-    creditAccount: '1600', // Trade payables
+    creditAccount: '70001', // Creditor account (supplier)
     amount: 119,
     description: 'Purchase including VAT',
     skrType: 'SKR03',