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/lib.rs

@@ -11,6 +11,7 @@ pub mod integrations;
 pub mod movies;
 pub mod person;
 pub mod search;
+pub mod social;
 pub mod users;
 pub mod watchlist;
 pub mod wrapup;