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/application/src/social/mod.rs

@@ -1,15 +1,4 @@
 pub mod commands;
 pub mod deps;
+pub mod execute;
 pub mod queries;
-
-pub mod accept;
-pub mod block;
-pub mod follow;
-pub mod get_blocked;
-pub mod get_followers;
-pub mod get_following;
-pub mod get_pending;
-pub mod reject;
-pub mod remove_follower;
-pub mod unblock;
-pub mod unfollow;