feat(poll): add filtered response lookup contract

src/govoplan_core/core/poll.py

@@ -222,6 +222,19 @@ class PollSchedulingProvider(Protocol):
     def list_responses(self, session: object, *, tenant_id: str, poll_id: str) -> Sequence[PollResponseRef]:
         ...
 
+    def get_response(
+        self,
+        session: object,
+        *,
+        tenant_id: str,
+        poll_id: str,
+        respondent_ids: Sequence[str],
+        invitation_id: str | None = None,
+    ) -> PollResponseRef | None:
+        """Return one response matching server-trusted participant identities."""
+
+        ...
+
 
 @runtime_checkable
 class PollResponseSubmissionProvider(Protocol):