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/services/AssetService.ts

@@ -0,0 +1,91 @@
+import {Asset, AssetType} from '@prisma/client';
+import {AssetRepository} from '../repositories/AssetRepository';
+import {NotFoundError, ForbiddenError, ValidationError} from '../utils/errors';
+
+interface CreateAssetDTO {
+  name: string;
+  type: AssetType;
+  value: number;
+}
+
+interface UpdateAssetDTO {
+  name?: string;
+  type?: AssetType;
+  value?: number;
+}
+
+/**
+ * Asset Service
+ * Implements Single Responsibility: Handles asset business logic
+ * Implements Open/Closed: Extensible for new asset-related features
+ */
+export class AssetService {
+  constructor(private assetRepository: AssetRepository) {}
+
+  async getAll(userId: string): Promise<Asset[]> {
+    return this.assetRepository.findAllByUser(userId);
+  }
+
+  async getById(id: string, userId: string): Promise<Asset> {
+    const asset = await this.assetRepository.findByIdAndUser(id, userId);
+    if (!asset) {
+      throw new NotFoundError('Asset not found');
+    }
+    return asset;
+  }
+
+  async create(userId: string, data: CreateAssetDTO): Promise<Asset> {
+    this.validateAssetData(data);
+
+    return this.assetRepository.create({
+      name: data.name,
+      type: data.type,
+      value: data.value,
+      user: {connect: {id: userId}},
+    });
+  }
+
+  async update(id: string, userId: string, data: UpdateAssetDTO): Promise<Asset> {
+    const asset = await this.assetRepository.findByIdAndUser(id, userId);
+    if (!asset) {
+      throw new NotFoundError('Asset not found');
+    }
+
+    if (data.value !== undefined || data.name !== undefined || data.type !== undefined) {
+      this.validateAssetData({
+        name: data.name || asset.name,
+        type: data.type || asset.type,
+        value: data.value !== undefined ? data.value : asset.value,
+      });
+    }
+
+    return this.assetRepository.update(id, data);
+  }
+
+  async delete(id: string, userId: string): Promise<void> {
+    const asset = await this.assetRepository.findByIdAndUser(id, userId);
+    if (!asset) {
+      throw new NotFoundError('Asset not found');
+    }
+
+    await this.assetRepository.delete(id);
+  }
+
+  async getTotalValue(userId: string): Promise<number> {
+    return this.assetRepository.getTotalValue(userId);
+  }
+
+  private validateAssetData(data: CreateAssetDTO): void {
+    if (!data.name || data.name.trim().length === 0) {
+      throw new ValidationError('Asset name is required');
+    }
+
+    if (data.value < 0) {
+      throw new ValidationError('Asset value cannot be negative');
+    }
+
+    if (!Object.values(AssetType).includes(data.type)) {
+      throw new ValidationError('Invalid asset type');
+    }
+  }
+}