refactor: consolidate 11 social use cases → SocialCmd/SocialQry enum dispatch

-280 net lines. 11 one-file use cases replaced by execute_command/
execute_query with enum dispatch. Added FollowRejected event (reject
was silently dropping). Command→event mapping now explicit in one match.

crates/domain/src/events.rs

@@ -71,6 +71,10 @@ pub enum DomainEvent {
         owner: UserId,
         requester: crate::value_objects::SocialIdentity,
     },
+    FollowRejected {
+        owner: UserId,
+        requester: crate::value_objects::SocialIdentity,
+    },
     Unfollowed {
         follower: UserId,
         target: crate::value_objects::SocialIdentity,