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__/jose.js

@@ -0,0 +1,13 @@
+module.exports = {
+  SignJWT: class {
+    constructor() { return this; }
+    setProtectedHeader() { return this; }
+    setIssuedAt() { return this; }
+    setExpirationTime() { return this; }
+    sign() { return Promise.resolve('mocked-token'); }
+  },
+  jwtVerify: () => Promise.resolve({ payload: { sub: 'mocked-user' } }),
+  importJWK: () => Promise.resolve({}),
+  exportJWK: () => Promise.resolve({}),
+  generateKeyPair: () => Promise.resolve({ publicKey: {}, privateKey: {} }),
+};