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.

ts/skr.classes.journalentry.ts

@@ -2,6 +2,11 @@ import * as plugins from './plugins.js';
 import { getDbSync } from './skr.database.js';
 import { Account } from './skr.classes.account.js';
 import { Transaction } from './skr.classes.transaction.js';
+import {
+  validatePostingKey,
+  validatePostingKeyConsistency,
+  getPostingKeyDescription,
+} from './skr.postingkeys.js';
 import type {
   TSKRType,
   IJournalEntry,
@@ -212,22 +217,84 @@ export class JournalEntry extends SmartDataDbDoc<JournalEntry, JournalEntry> {
       throw new Error('Journal entry must have at least 2 lines');
     }
 
-    // Validate all accounts exist and are active
+    // Validate all accounts exist, are active, and can be posted to
+    const validationErrors: string[] = [];
+    const validationWarnings: string[] = [];
+
     for (const line of this.lines) {
+      // Validate posting key is present (REQUIRED)
+      if (!line.postingKey) {
+        validationErrors.push(
+          `Line for account ${line.accountNumber} is missing required posting key (Buchungsschlüssel). ` +
+          `Posting keys are mandatory for DATEV compliance.`
+        );
+        continue; // Skip further validation for this line
+      }
+
+      // Validate account is not an automatic account (Automatikkonto)
+      try {
+        await Account.validateAccountForPosting(line.accountNumber, this.skrType);
+      } catch (error) {
+        validationErrors.push(error.message);
+        continue; // Skip further validation for this line
+      }
+
+      // Get account for posting key validation
       const account = await Account.getAccountByNumber(
         line.accountNumber,
         this.skrType,
       );
 
       if (!account) {
-        throw new Error(
+        validationErrors.push(
           `Account ${line.accountNumber} not found for ${this.skrType}`,
         );
+        continue;
       }
 
       if (!account.isActive) {
-        throw new Error(`Account ${line.accountNumber} is not active`);
+        validationErrors.push(`Account ${line.accountNumber} is not active`);
+        continue;
       }
+
+      // Validate posting key for this line
+      const amount = line.debit || line.credit || 0;
+      const postingKeyValidation = validatePostingKey(
+        line.postingKey,
+        line.accountNumber,
+        amount
+      );
+
+      if (!postingKeyValidation.isValid) {
+        validationErrors.push(...postingKeyValidation.errors);
+      }
+
+      if (postingKeyValidation.warnings.length > 0) {
+        validationWarnings.push(...postingKeyValidation.warnings);
+      }
+    }
+
+    // Validate posting key consistency across all lines
+    const consistencyValidation = validatePostingKeyConsistency(this.lines);
+    if (!consistencyValidation.isValid) {
+      validationErrors.push(...consistencyValidation.errors);
+    }
+    if (consistencyValidation.warnings.length > 0) {
+      validationWarnings.push(...consistencyValidation.warnings);
+    }
+
+    // Log warnings but don't fail validation
+    if (validationWarnings.length > 0) {
+      console.warn('Journal entry validation warnings:');
+      validationWarnings.forEach(warning => console.warn(`  - ${warning}`));
+    }
+
+    // Throw if any errors
+    if (validationErrors.length > 0) {
+      throw new Error(
+        'Journal entry validation failed:\n' +
+        validationErrors.map(e => `  - ${e}`).join('\n')
+      );
     }
   }
 
@@ -325,6 +392,7 @@ export class JournalEntry extends SmartDataDbDoc<JournalEntry, JournalEntry> {
       credit: line.debit, // Swap
       description: `Reversal: ${line.description || ''}`,
       costCenter: line.costCenter,
+      postingKey: line.postingKey, // Keep same posting key for reversal
     }));
 
     const reversalEntry = new JournalEntry({