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 7 user CRUD methods into sqlmodel_repo/auth.py.
_ensure_admin_user stays in __init__.py so DECNET_ADMIN_PASSWORD
remains addressable at the module path tests already monkeypatch.
Moves upsert_attacker_intel, get_attacker_intel_by_uuid,
and get_unenriched_attackers into sqlmodel_repo/attacker_intel.py.
Composed onto SQLModelRepository via mixin inheritance.
Pure rename — the old monolithic 3505-line file becomes
decnet/web/db/sqlmodel_repo/__init__.py. No code changes.
Subsequent commits will extract per-domain mixins out of __init__.py
to mirror the topical layout used by decnet/web/db/models/.
