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/middleware/auth.ts

@@ -0,0 +1,36 @@
+import {FastifyRequest, FastifyReply} from 'fastify';
+import {UnauthorizedError} from '../utils/errors';
+
+/**
+ * Extend Fastify Request with user property
+ */
+declare module 'fastify' {
+  interface FastifyRequest {
+    user?: {
+      id: string;
+      email: string;
+    };
+  }
+}
+
+/**
+ * Authentication Middleware
+ * Verifies JWT token and attaches user to request
+ */
+export async function authenticate(request: FastifyRequest, reply: FastifyReply) {
+  try {
+    await request.jwtVerify();
+  } catch (err) {
+    throw new UnauthorizedError('Invalid or expired token');
+  }
+}
+
+/**
+ * Extract user ID from authenticated request
+ */
+export function getUserId(request: FastifyRequest): string {
+  if (!request.user || !request.user.id) {
+    throw new UnauthorizedError('User not authenticated');
+  }
+  return request.user.id;
+}

backend-api/src/middleware/errorHandler.ts

@@ -0,0 +1,64 @@
+import {FastifyError, FastifyReply, FastifyRequest} from 'fastify';
+import {AppError} from '../utils/errors';
+import {ZodError} from 'zod';
+
+/**
+ * Global Error Handler
+ * Implements Single Responsibility: Handles all error responses
+ */
+export async function errorHandler(error: FastifyError, request: FastifyRequest, reply: FastifyReply) {
+  // Log error
+  request.log.error(error);
+
+  // Handle custom app errors
+  if (error instanceof AppError) {
+    return reply.status(error.statusCode).send({
+      error: error.name,
+      message: error.message,
+    });
+  }
+
+  // Handle Zod validation errors
+  if (error instanceof ZodError) {
+    return reply.status(400).send({
+      error: 'ValidationError',
+      message: 'Invalid request data',
+      details: error.errors,
+    });
+  }
+
+  // Handle Fastify validation errors
+  if (error.validation) {
+    return reply.status(400).send({
+      error: 'ValidationError',
+      message: error.message,
+      details: error.validation,
+    });
+  }
+
+  // Handle Prisma errors
+  if (error.name === 'PrismaClientKnownRequestError') {
+    const prismaError = error as any;
+    if (prismaError.code === 'P2002') {
+      return reply.status(409).send({
+        error: 'ConflictError',
+        message: 'A record with this value already exists',
+      });
+    }
+    if (prismaError.code === 'P2025') {
+      return reply.status(404).send({
+        error: 'NotFoundError',
+        message: 'Record not found',
+      });
+    }
+  }
+
+  // Default server error
+  const statusCode = error.statusCode || 500;
+  const message = statusCode === 500 ? 'Internal server error' : error.message;
+
+  return reply.status(statusCode).send({
+    error: 'ServerError',
+    message,
+  });
+}