fix(dlq): preserve parallel queue type when adding jobs to DLQ

[rrogers-machinify]

Summary

• Fix parallel jobs being converted to serial when requeued from DLQ
• Use empty string instead of "default" for parallel jobs in DLQ

Problem

When parallel jobs (Queue::Parallel) fail and are added to the DLQ, they were being stored with queue_name = "default". When requeued, the requeue logic sees a non-empty string and creates Queue::Serial("default") instead of Queue::Parallel.

The requeue logic already correctly handles this distinction:

let queue = if dlq_job.queue_name.is_empty() {
    Queue::Parallel      // ← Empty string = parallel
} else {
    Queue::Serial(dlq_job.queue_name.clone())  // ← Non-empty = serial
};

But add_to_dlq() and process_failed_jobs() were defaulting to "default" instead of empty string for parallel jobs (which have no job_queue_id).

Fix

• add_to_dlq(): Use String::new() when job_queue_id() is None (parallel job)
• process_failed_jobs(): Use unwrap_or_default() when queue_name is NULL

This ensures parallel jobs stay parallel when requeued from DLQ.

Test plan

• cargo check passes
• just fmt - no changes needed

🤖 Generated with Claude Code

[ceejbot]

That was a case I missed, sorry. Good catch.