fix: make audiobook search retries fair and deduplicated

[galail52]

Summary

• Retry oldest or never-searched audiobook requests first.
• Prevent duplicate queued or active audiobook searches for the same request.
• Use an owner token so an older job cannot clear a newer search lock after TTL expiry.
• Keep the existing retry schedule and batch size unchanged.

Validation

• npm test - 2620 passed
• npm run build - passed
• docker compose build readmeabook - command exits successfully; the current compose file reports no services to build