Fix scheduler retry backoff with jitter

[Thanhdn1984]

Summary

• Add scheduled retry backoff with jitter for failed queue tasks instead of immediate requeue.
• Preserve task id/retry count/priority/queue while retrying.
• Fix scheduled task bookkeeping so delayed tasks store task metadata, not only a timestamp.
• Add regression coverage for retry delay behavior.

Validation

• python3 -m py_compile src/orchestrator/scheduler.py
• pytest tests/test_scheduler.py -q currently blocked by an existing unrelated import error: src.orchestrator.__init__ imports AgentStatus from src.agent, but src.agent does not export it.

Bounty target: #3929