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.

Cargo.lock

@@ -2554,6 +2554,7 @@ dependencies = [
  "async-trait",
  "chrono",
  "k-ap",
+ "serde_json",
  "sqlx",
  "tokio",
  "tracing",