Add per-app AP social UI for blog, market, and events

Lightweight social pages (search, follow/unfollow, followers, following,
actor timeline) auto-registered for AP-enabled apps via shared blueprint.
Federation keeps the full social hub. Followers scoped per app_domain;
post cards show "View on Hub" link instead of interaction buttons.

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

shared/contracts/protocols.py

@@ -243,6 +243,7 @@ class FederationService(Protocol):
     async def get_followers_paginated(
         self, session: AsyncSession, username: str,
         page: int = 1, per_page: int = 20,
+        app_domain: str | None = None,
     ) -> tuple[list[RemoteActorDTO], int]: ...
 
     async def add_follower(