feat: Enhance Booklooker client with webhook signature verification, idempotency handling, and retry logic

tests/test_webhooks.py

@@ -1,4 +1,4 @@
-from booklooker_client import BooklookerWebhookHelper
+from booklooker_client import BooklookerWebhookHelper, InMemoryIdempotencyStore
 from booklooker_client.models.order import OrderBatch, OrderRecord
 
 
@@ -39,3 +39,9 @@ def test_duplicate_detection() -> None:
     assert first.resource_type == "order"
     assert second.resource_type == "duplicate"
     assert second.enriched_data == {"duplicate": True}
+
+
+def test_idempotency_store_ttl_expiry() -> None:
+    store = InMemoryIdempotencyStore(ttl_seconds=0)
+    store.mark_seen("evt-expire")
+    assert store.has_seen("evt-expire") is False