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__/sha3.js

@@ -0,0 +1,5 @@
+module.exports = {
+  sha3_256: () => ({ update: () => ({ digest: () => Buffer.alloc(32) }) }),
+  sha3_512: () => ({ update: () => ({ digest: () => Buffer.alloc(64) }) }),
+  shake256: () => ({ update: () => ({ digest: () => Buffer.alloc(32) }) }),
+};