feat: add PATCH /federation/me/also-known-as endpoint

Adds alsoKnownAs column to users table (migration 013), reads it in
the AP actor JSON, and exposes PATCH /federation/me/also-known-as to
set or clear it. Required pre-condition for broadcast_move.

crates/domain/src/testing/mod.rs

@@ -152,6 +152,14 @@ impl UserWriter for TestStore {
         }
         Ok(())
     }
+
+    async fn set_also_known_as(
+        &self,
+        _user_id: &UserId,
+        _value: Option<String>,
+    ) -> Result<(), DomainError> {
+        Ok(())
+    }
 }
 
 #[async_trait]