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/.env.example

@@ -0,0 +1,6 @@
+NODE_ENV=development
+PORT=3000
+DATABASE_URL="postgresql://user:password@localhost:5432/personal_finances?schema=public"
+JWT_SECRET=your-secret-key-change-this-in-production
+JWT_EXPIRES_IN=7d
+CORS_ORIGIN=http://localhost:5174