Add DomainThrottle for per-domain rate limiting

Implement per-domain throttling for purgatory sync operations:
- Concurrent request limit (max in-flight requests per domain)
- Rate limit (max requests per minute via sliding window)
- Fair round-robin queue processing across identifiers
- In-progress tracking to prevent duplicate fetches
- Tried URL tracking per identifier

Add indexmap dependency for ordered iteration in round-robin queue.

Includes 6 unit tests covering:
- Concurrent limit enforcement
- Rate limit enforcement (sliding window)
- Round-robin fair processing
- In-progress identifier skipping
- Round-robin index adjustment on removal
- Tried URL merging on re-enqueue

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