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/adapters/nats/src/subject.rs

@@ -14,6 +14,7 @@ pub fn event_to_subject(prefix: &str, event: &DomainEvent) -> String {
         DomainEvent::WatchlistEntryRemoved { .. } => "watchlist.entry.removed",
         DomainEvent::FollowRequested { .. } => "follow.requested",
         DomainEvent::FollowAccepted { .. } => "follow.accepted",
+        DomainEvent::FollowRejected { .. } => "follow.rejected",
         DomainEvent::Unfollowed { .. } => "follow.unfollowed",
         DomainEvent::FollowerRemoved { .. } => "follower.removed",
         DomainEvent::ActorBlocked { .. } => "actor.blocked",