[Backend] Create background job queue for async tasks

[SYLVIANNORUKA]

Description: Implement a durable job queue to move long-running tasks such as email sending and transcoding off the request path.

Requirements:

• Handle job enqueue requests
• Track job status through processing states
• Validate job payload schema before enqueue
• Manage retry with exponential backoff on failure
• Provide job status query endpoint

Acceptance Criteria:

• Enqueue requests are handled
• Job status is tracked through states
• Payload schema is validated
• Retry with backoff works
• Status query endpoint is provided

Technical Details:

Files: backend/src/modules/../common/queue/queue.module.ts, backend/src/modules/notifications/notifications.service.ts
Libraries: bullmq, ioredis, @nestjs/bullmq