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/repositories/LiabilityRepository.ts

@@ -0,0 +1,73 @@
+import {Liability, Prisma} from '@prisma/client';
+import {DatabaseConnection} from '../config/database';
+import {IUserScopedRepository} from './interfaces/IRepository';
+
+const prisma = DatabaseConnection.getInstance();
+
+/**
+ * Repository for Liability data access
+ * Implements Single Responsibility Principle - handles only database operations
+ */
+export class LiabilityRepository implements IUserScopedRepository<Liability> {
+  async findById(id: string): Promise<Liability | null> {
+    return prisma.liability.findUnique({
+      where: {id},
+    });
+  }
+
+  async findAllByUser(userId: string): Promise<Liability[]> {
+    return prisma.liability.findMany({
+      where: {userId},
+      orderBy: {createdAt: 'desc'},
+    });
+  }
+
+  async create(data: Prisma.LiabilityCreateInput): Promise<Liability> {
+    return prisma.liability.create({
+      data,
+    });
+  }
+
+  async update(id: string, data: Prisma.LiabilityUpdateInput): Promise<Liability> {
+    return prisma.liability.update({
+      where: {id},
+      data,
+    });
+  }
+
+  async delete(id: string): Promise<void> {
+    await prisma.liability.delete({
+      where: {id},
+    });
+  }
+
+  /**
+   * Get total value of all liabilities for a user
+   */
+  async getTotalValue(userId: string): Promise<number> {
+    const result = await prisma.liability.aggregate({
+      where: {userId},
+      _sum: {
+        currentBalance: true,
+      },
+    });
+
+    return result._sum.currentBalance || 0;
+  }
+
+  /**
+   * Get liabilities grouped by type
+   */
+  async getByType(userId: string): Promise<Record<string, Liability[]>> {
+    const liabilities = await this.findAllByUser(userId);
+
+    return liabilities.reduce((acc, liability) => {
+      const type = liability.type;
+      if (!acc[type]) {
+        acc[type] = [];
+      }
+      acc[type].push(liability);
+      return acc;
+    }, {} as Record<string, Liability[]>);
+  }
+}