Backend already has `@nestjs/testing` but most tests instantiate services with no mocks

[mercy60]

Internal ID: #95

Imported from ISSUES.md via scripts/import-issues.mjs.

Problem Statement. quiz.service.spec.ts uses Test.createTestingModule({ providers: [QuizService] }) — this leaves QuizService partially constructed because its dependencies (manual injections) are not provided.

Expected Outcome.

• Each spec builds a complete module with mocks.

Acceptance Criteria.

• Tests succeed independently.

Files / modules affected.

• All spec files.

Dependencies. None.

Difficulty / Effort. Medium / L (≈ 1 week).

Labels. testing