Add backend API for personal finance management application

- Introduced a comprehensive backend API using TypeScript, Fastify, and PostgreSQL.
- Added essential files including architecture documentation, environment configuration, and Docker setup.
- Implemented RESTful routes for managing assets, liabilities, clients, invoices, and cashflow.
- Established a robust database schema with Prisma for data management.
- Integrated middleware for authentication and error handling.
- Created service and repository layers to adhere to SOLID principles and clean architecture.
- Included example environment variables for development, staging, and production setups.

backend-api/src/utils/errors.ts

@@ -0,0 +1,38 @@
+/**
+ * Custom error classes
+ * Implements Open/Closed Principle: Extensible for new error types
+ */
+
+export abstract class AppError extends Error {
+  abstract statusCode: number;
+
+  constructor(message: string) {
+    super(message);
+    this.name = this.constructor.name;
+    Error.captureStackTrace(this, this.constructor);
+  }
+}
+
+export class NotFoundError extends AppError {
+  statusCode = 404;
+}
+
+export class ValidationError extends AppError {
+  statusCode = 400;
+}
+
+export class UnauthorizedError extends AppError {
+  statusCode = 401;
+}
+
+export class ForbiddenError extends AppError {
+  statusCode = 403;
+}
+
+export class ConflictError extends AppError {
+  statusCode = 409;
+}
+
+export class InternalServerError extends AppError {
+  statusCode = 500;
+}