feat: add profile fields for local users

DB→domain→API→AP→frontend end-to-end. Fields stored as
JSONB, exposed via PATCH /users/me, serialized as AP
PropertyValue attachment. Editor in federation settings,
display on profile card.

crates/application/src/testing.rs

@@ -73,6 +73,7 @@ impl ActivityPubRepository for TestApRepo {
             avatar_url: None,
             header_url: None,
             custom_css: None,
+            profile_fields: vec![],
             local: false,
             created_at: chrono::Utc::now(),
             updated_at: chrono::Utc::now(),