fix(backend): restore Schedule responses in ToolResponseUnion after master sync

Conflict resolution dropped ScheduleListResponse and ScheduleDeletedResponse
from the codegen union. Both classes still exist in manage_schedules.py and
are returned at runtime, and openapi.json was not regenerated as part of the
merge — so the platform-fullstack-ci export-api-schema gate would fail when
the regenerated file diffs against the committed snapshot.

Re-import and re-add the two arms; verified pnpm format + export-api-schema
round-trip produces zero diff against the committed openapi.json.