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/services/post-quantum.service.ts

@@ -0,0 +1,20 @@
+import { Injectable } from "@nestjs/common";
+import { ml_kem768 } from "@noble/post-quantum/ml-kem.js";
+
+@Injectable()
+export class PostQuantumService {
+	generatePostQuantumKeyPair() {
+		const seed = new Uint8Array(64);
+		crypto.getRandomValues(seed);
+		const { publicKey, secretKey } = ml_kem768.keygen(seed);
+		return { publicKey, secretKey };
+	}
+
+	encapsulate(publicKey: Uint8Array) {
+		return ml_kem768.encapsulate(publicKey);
+	}
+
+	decapsulate(cipherText: Uint8Array, secretKey: Uint8Array) {
+		return ml_kem768.decapsulate(cipherText, secretKey);
+	}
+}