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.types.ts

@@ -9,6 +9,18 @@ export type TSKRType = 'SKR03' | 'SKR04';
 
 export type TTransactionStatus = 'pending' | 'posted' | 'reversed';
 
+/**
+ * DATEV posting keys (Buchungsschlüssel) for German accounting
+ * These keys control automatic VAT booking and are checked in tax audits
+ */
+export type TPostingKey =
+  | 3   // Payment with 19% VAT
+  | 8   // 7% input VAT
+  | 9   // 19% input VAT
+  | 19  // 19% input VAT (intra-EU)
+  | 40  // Tax-free (disables VAT automatism)
+  | 94; // 19% input/output VAT (reverse charge)
+
 export type TReportType =
   | 'trial_balance'
   | 'income_statement'
@@ -16,6 +28,18 @@ export type TReportType =
   | 'general_ledger'
   | 'cash_flow';
 
+/**
+ * Posting key validation rule
+ */
+export interface IPostingKeyRule {
+  key: TPostingKey;
+  description: string;
+  vatRate?: number;              // Expected VAT rate (if applicable)
+  requiresVAT: boolean;           // Whether VAT entry is required
+  disablesVATAutomatism: boolean; // Whether this key disables automatic VAT
+  allowedScenarios?: string[];    // Allowed tax scenarios (e.g., 'reverse_charge')
+}
+
 export interface IAccountData {
   accountNumber: string;
   accountName: string;
@@ -25,6 +49,7 @@ export interface IAccountData {
   description?: string;
   vatRate?: number;
   isActive?: boolean;
+  isAutomaticAccount?: boolean; // Automatikkonto (e.g., 1400, 1600) - cannot be posted to directly
 }
 
 export interface ITransactionData {
@@ -53,6 +78,7 @@ export interface IJournalEntryLine {
   credit?: number;
   description?: string;
   costCenter?: string;
+  postingKey: TPostingKey; // REQUIRED: DATEV posting key for VAT automation control
 }
 
 export interface ITrialBalanceEntry {