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__/cuid2.js

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+module.exports = {
+  createCuid: () => () => 'mocked-cuid',
+};

backend/test/__mocks__/jose.js

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+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: {} }),
+};

backend/test/__mocks__/ml-kem.js

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+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),
+  }
+};

backend/test/__mocks__/sha3.js

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