[BE-125] Unit tests for the queue module

[mftee]

Overview

backend/src/queue/ has 3 files and no tests. bullmq, @nestjs/bull, and ioredis are installed, so background processing carries real work — and a silently failing job is invisible without tests or observability.

Tasks

• Test job producers enqueue with the expected name, payload, and options.
• Test consumers process correctly and handle malformed payloads without crashing the worker.
• Test retry behaviour and that a permanently failing job ends in a defined state rather than vanishing.
• Test idempotency — the same job processed twice must not double-apply its effect.
• Test that job payloads carry no secrets, since they persist in Redis.
• Mock Redis rather than requiring a live instance.

Acceptance Criteria

• npm run test -- queue passes without a running Redis.
• A malformed payload cannot crash the worker.
• Duplicate processing is proven safe.

Notes for Contributors

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