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.
Module-level session helpers move into sqlmodel_repo/_helpers.py. __init__.py re-exports them so external import paths (decnet.web.db.sqlmodel_repo._safe_session) keep resolving.
