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/common/interfaces/mail.interface.ts

@@ -0,0 +1,4 @@
+export interface IMailService {
+	sendEmailValidation(email: string, token: string): Promise<void>;
+	sendPasswordReset(email: string, token: string): Promise<void>;
+}