refactor: SocialActor rich queries, migrate remaining handlers, slim SocialQueryPort

SocialQuery returns SocialActor (identity+handle+display_name+avatar_url)
instead of bare SocialIdentity. Migrated get_following_page,
get_followers_page, get_blocked_actors_page to use cases. Moved
get_activity_feed + get_profile from SocialQueryPort to SocialQuery.
Legacy SocialQueryPort remains only for get_users listing.

crates/domain/src/value_objects/social.rs

@@ -15,3 +15,11 @@ impl SocialIdentity {
         matches!(self, Self::Remote { .. })
     }
 }
+
+#[derive(Clone, Debug)]
+pub struct SocialActor {
+    pub identity: SocialIdentity,
+    pub handle: String,
+    pub display_name: Option<String>,
+    pub avatar_url: Option<String>,
+}