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/adapters/postgres/migrations/020_user_profile_fields.sql

@@ -0,0 +1 @@
+ALTER TABLE users ADD COLUMN IF NOT EXISTS profile_fields JSONB DEFAULT '[]'::jsonb;