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>

contracts/protocols.py

@@ -262,6 +262,10 @@ class FederationService(Protocol):
         self, session: AsyncSession, acct: str,
     ) -> RemoteActorDTO | None: ...
 
+    async def search_actors(
+        self, session: AsyncSession, query: str, page: int = 1, limit: int = 20,
+    ) -> tuple[list[RemoteActorDTO], int]: ...
+
     # -- Following (outbound) -------------------------------------------------
     async def send_follow(
         self, session: AsyncSession, local_username: str, remote_actor_url: str,