feat: add modular services and repositories for improved code organization

Introduce repository pattern across multiple services, including `favorites`, `tags`, `sessions`, `reports`, `auth`, and more. Decouple crypto functionalities into modular services like `HashingService`, `JwtService`, and `EncryptionService`. Improve testability and maintainability by simplifying dependencies and consolidating utility logic.

backend/src/mail/mail.service.ts

@@ -1,9 +1,10 @@
 import { Injectable, Logger } from "@nestjs/common";
 import { ConfigService } from "@nestjs/config";
 import { MailerService } from "@nestjs-modules/mailer";
+import type { IMailService } from "../common/interfaces/mail.interface";
 
 @Injectable()
-export class MailService {
+export class MailService implements IMailService {
 	private readonly logger = new Logger(MailService.name);
 	private readonly domain: string;