Moves the 6 fleet-decky methods (incl. cross-source list_running_deckies aggregator) into sqlmodel_repo/fleet.py. _serialize_json_fields and _deserialize_json_fields move to _helpers.py since they're shared across fleet, topology, and canary.
Moves the 6 fleet-decky methods (incl. cross-source list_running_deckies aggregator) into sqlmodel_repo/fleet.py. _serialize_json_fields and _deserialize_json_fields move to _helpers.py since they're shared across fleet, topology, and canary.