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/test/__mocks__/ml-kem.js

@@ -0,0 +1,7 @@
+module.exports = {
+  ml_kem768: {
+    keygen: () => ({ publicKey: Buffer.alloc(1184), secretKey: Buffer.alloc(2400) }),
+    encapsulate: () => ({ cipherText: Buffer.alloc(1088), sharedSecret: Buffer.alloc(32) }),
+    decapsulate: () => Buffer.alloc(32),
+  }
+};