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

1 files changed, 3 insertions, 0 deletions

diff --git a/Cargo.toml b/Cargo.toml
index e1e1627..eceebcb 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -33,6 +33,9 @@ prometheus = "0.13"
 dashmap = "5"
 lazy_static = "1.4"
 
+# Data structures
+indexmap = "2"
+
 # Random (for startup jitter)
 rand = "0.8"