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/services/stubs.py

@@ -28,5 +28,15 @@ class StubFederationService:
     async def count_activities_for_source(self, session, source_type, source_id, *, activity_type):
         return 0
 
+    async def get_followers_paginated(self, session, username,
+                                      page=1, per_page=20, app_domain=None):
+        return [], 0
+
+    async def get_following(self, session, username, page=1, per_page=20):
+        return [], 0
+
+    async def search_actors(self, session, query, page=1, limit=20):
+        return [], 0
+
     async def get_stats(self, session):
         return {"actors": 0, "activities": 0, "followers": 0}