feat(core): initial release of SKR03/SKR04 German accounting standards implementation

- Complete implementation of German standard charts of accounts
- SKR03 (Process Structure Principle) for trading/service companies
- SKR04 (Financial Classification Principle) for manufacturing companies
- Double-entry bookkeeping with MongoDB persistence
- Comprehensive reporting suite with DATEV export
- Full TypeScript support and type safety

ts/skr.types.ts

@@ -0,0 +1,154 @@
+export type TAccountType =
+  | 'asset'
+  | 'liability'
+  | 'equity'
+  | 'revenue'
+  | 'expense';
+
+export type TSKRType = 'SKR03' | 'SKR04';
+
+export type TTransactionStatus = 'pending' | 'posted' | 'reversed';
+
+export type TReportType =
+  | 'trial_balance'
+  | 'income_statement'
+  | 'balance_sheet'
+  | 'general_ledger'
+  | 'cash_flow';
+
+export interface IAccountData {
+  accountNumber: string;
+  accountName: string;
+  accountClass: number;
+  accountType: TAccountType;
+  skrType: TSKRType;
+  description?: string;
+  vatRate?: number;
+  isActive?: boolean;
+}
+
+export interface ITransactionData {
+  date: Date;
+  debitAccount: string;
+  creditAccount: string;
+  amount: number;
+  description: string;
+  reference?: string;
+  skrType: TSKRType;
+  vatAmount?: number;
+  costCenter?: string;
+}
+
+export interface IJournalEntry {
+  date: Date;
+  description: string;
+  reference?: string;
+  lines: IJournalEntryLine[];
+  skrType: TSKRType;
+}
+
+export interface IJournalEntryLine {
+  accountNumber: string;
+  debit?: number;
+  credit?: number;
+  description?: string;
+  costCenter?: string;
+}
+
+export interface ITrialBalanceEntry {
+  accountNumber: string;
+  accountName: string;
+  debitBalance: number;
+  creditBalance: number;
+  netBalance: number;
+}
+
+export interface ITrialBalanceReport {
+  date: Date;
+  skrType: TSKRType;
+  entries: ITrialBalanceEntry[];
+  totalDebits: number;
+  totalCredits: number;
+  isBalanced: boolean;
+}
+
+export interface IIncomeStatementEntry {
+  accountNumber: string;
+  accountName: string;
+  amount: number;
+  percentage?: number;
+}
+
+export interface IIncomeStatement {
+  date: Date;
+  skrType: TSKRType;
+  revenue: IIncomeStatementEntry[];
+  expenses: IIncomeStatementEntry[];
+  totalRevenue: number;
+  totalExpenses: number;
+  netIncome: number;
+}
+
+export interface IBalanceSheetEntry {
+  accountNumber: string;
+  accountName: string;
+  amount: number;
+}
+
+export interface IBalanceSheet {
+  date: Date;
+  skrType: TSKRType;
+  assets: {
+    current: IBalanceSheetEntry[];
+    fixed: IBalanceSheetEntry[];
+    totalAssets: number;
+  };
+  liabilities: {
+    current: IBalanceSheetEntry[];
+    longTerm: IBalanceSheetEntry[];
+    totalLiabilities: number;
+  };
+  equity: {
+    entries: IBalanceSheetEntry[];
+    totalEquity: number;
+  };
+  isBalanced: boolean;
+}
+
+export interface IAccountFilter {
+  skrType?: TSKRType;
+  accountClass?: number;
+  accountType?: TAccountType;
+  isActive?: boolean;
+  searchTerm?: string;
+}
+
+export interface ITransactionFilter {
+  skrType?: TSKRType;
+  dateFrom?: Date;
+  dateTo?: Date;
+  accountNumber?: string;
+  minAmount?: number;
+  maxAmount?: number;
+  searchTerm?: string;
+}
+
+export interface IDatabaseConfig {
+  mongoDbUrl: string;
+  dbName?: string;
+}
+
+export interface IReportParams {
+  dateFrom?: Date;
+  dateTo?: Date;
+  skrType: TSKRType;
+  format?: 'json' | 'csv' | 'datev';
+}
+
+export interface IAccountBalance {
+  accountNumber: string;
+  debitTotal: number;
+  creditTotal: number;
+  balance: number;
+  lastUpdated: Date;
+}