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/crypto/crypto.service.spec.ts

@@ -64,6 +64,10 @@ jest.mock("jose", () => ({
 }));
 
 import { CryptoService } from "./crypto.service";
+import { HashingService } from "./services/hashing.service";
+import { JwtService } from "./services/jwt.service";
+import { EncryptionService } from "./services/encryption.service";
+import { PostQuantumService } from "./services/post-quantum.service";
 
 describe("CryptoService", () => {
 	let service: CryptoService;
@@ -72,6 +76,10 @@ describe("CryptoService", () => {
 		const module: TestingModule = await Test.createTestingModule({
 			providers: [
 				CryptoService,
+				HashingService,
+				JwtService,
+				EncryptionService,
+				PostQuantumService,
 				{
 					provide: ConfigService,
 					useValue: {