Add search_actors to FederationService for paginated actor search

Fuzzy ILIKE search across remote actors and local profiles, with
WebFinger resolution for @user@domain queries. Supports page-based
pagination for infinite scroll.

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

services/stubs.py

@@ -246,6 +246,9 @@ class StubFederationService:
     async def search_remote_actor(self, session, acct):
         return None
 
+    async def search_actors(self, session, query, page=1, limit=20):
+        return [], 0
+
     async def send_follow(self, session, local_username, remote_actor_url):
         raise RuntimeError("FederationService not available")