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.

env.example

@@ -0,0 +1,19 @@
+# Development (defaults work out of the box)
+DEV_DATABASE_URL=postgresql://wealth:wealth_dev@localhost:5432/wealth_dev
+
+# Staging
+STAGING_DB_PASSWORD=change-me-staging
+STAGING_DATABASE_URL=postgresql://wealth:change-me-staging@localhost:5433/wealth_staging
+
+# Production (required - no default)
+PROD_DB_PASSWORD=change-me-production
+PROD_DATABASE_URL=postgresql://wealth:change-me-production@localhost:5434/wealth_prod
+
+# JWT Secrets
+JWT_SECRET=change-me-jwt-secret-min-32-chars
+JWT_REFRESH_SECRET=change-me-refresh-secret-min-32-chars
+
+# App
+NODE_ENV=development
+PORT=3000
+