fix: recover expired Redis task reservations

[suguanYang]

Summary

• add a 30-second Celery Beat recovery task for expired Redis reservations
• invoke Kombu restore_visible(interval=1) through a fresh broker connection
• guard duplicate Beat firings and close broker resources safely
• add worker contract coverage

Evidence

• staging had two expired reservations in Redis after the configured 4,500-second visibility window
• an approved one-time Kombu restore requeued both reservations
• staging then redelivered the parse task and billing idempotency reported Job already charged
• local Celery 5.5.3/Kombu 5.5.4 gevent drills confirmed the reservation format and recovery behavior

Verification

• focused worker contracts: 8 passed
• Ruff: passed
• git diff --check: passed

This PR is review-only. Do not merge, promote to staging, deploy, or rerun the test job without explicit approval.