_ensure_admin_user was strict insert-if-missing: once a stale hash landed
in decnet.db (e.g. from a deploy that used a different DECNET_ADMIN_PASSWORD),
login silently 401'd because changing the env var later had no effect.
Now on startup: if the admin still has must_change_password=True (they
never finalized their own password), re-sync the hash from the current
env var. Once the admin sets a real password, we leave it alone.
Found via locustfile.py login storm — see tests/test_admin_seed.py.
Note: this commit also bundles uncommitted pool-management work already
present in sqlmodel_repo.py from prior sessions.
