Person struct now deserializes gracefully when outbox, followers, or
following are absent (Threads omits them for some actors). Accepts
Service/Application/Organization/Group in addition to Person.
manually_approves_followers defaults to false when absent.
Add display_name field to DbActor, populated from AP Person.name in
from_json. Expose LookedUpActor type and lookup_actor_by_handle method
on ActivityPubService — uses the existing signed webfinger_https path
so strict instances (Threads, etc.) return full actor data.
