Add per-app ActivityPub actors via shared AP blueprint

Each AP-enabled app (blog, market, events, federation) now serves its
own webfinger, actor profile, inbox, outbox, and followers endpoints.
Per-app actors are virtual projections of the same ActorProfile/keypair,
scoped by APFollower.app_domain and APActivity.origin_app.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

contracts/dtos.py

@@ -176,6 +176,7 @@ class APFollowerDTO:
     follower_inbox: str
     follower_actor_url: str
     created_at: datetime | None = None
+    app_domain: str | None = None
 
 
 @dataclass(frozen=True, slots=True)