feat: handle pull_request assigned action

server.py

@@ -429,6 +429,13 @@ async def github(request):
                     repository=repository,
                 )
 
+            elif action == "assigned":
+                assigned_user = request.json["assignee"]["login"]
+                await notify(
+                    f"[{repository}] @{user} {action} {assigned_user} on [pull request #{pull_request_number}]({url}): {pull_request_title}",
+                    repository=repository,
+                )
+
             elif action in (
                 "review_requested",
                 "review_request_removed",