feat: unified social identity layer — SocialIdentity, ports, use cases, adapter

SocialIdentity value object (Local|Remote), SocialCommand/SocialQuery
domain ports, 11 CQRS use cases w/ 14 tests, CompositeSocialAdapter
wrapping k_ap, 6 new domain events, handlers migrated from ap_service
to use cases. Closes #12 foundation — AP handlers + legacy cleanup TBD.

crates/application/src/social/mod.rs

@@ -0,0 +1,15 @@
+pub mod commands;
+pub mod deps;
+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;