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, 1 insertions, 0 deletions

diff --git a/Cargo.lock b/Cargo.lock
index fbcc015..03afcbb 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1435,6 +1435,7 @@ dependencies = [
  "http-body-util",
  "hyper 1.8.1",
  "hyper-util",
+ "indexmap",
  "lazy_static",
  "nostr-lmdb",
  "nostr-relay-builder",