fix(env): fail closed on weak DECNET_ADMIN_PASSWORD

DECNET_ADMIN_PASSWORD defaulted to the literal "admin" with no guard, so
a master that never set it seeded an admin/admin account. Resolve it
lazily via __getattr__ -> _require_env (the same pattern as
DECNET_JWT_SECRET): unset or a known-bad default (admin/secret/...) is
rejected, and <12 chars is rejected outside DECNET_DEVELOPER. Only the
master web/api processes that import the DB layer resolve it; workers
never do, and the pytest short-circuit keeps the dev loop unaffected.
The module attribute stays addressable for the admin-seed monkeypatch.

tests/core/test_env_secrets.py

@@ -0,0 +1,66 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+"""Fail-closed validation of security-sensitive env secrets (env._require_env).
+
+DECNET_ADMIN_PASSWORD must never silently default to "admin": it is resolved
+lazily and validated like DECNET_JWT_SECRET. These tests drive _require_env
+against a controlled environ so the production raise paths (which are bypassed
+under live pytest) are actually exercised.
+"""
+from __future__ import annotations
+
+import pytest
+
+import decnet.env as envmod
+
+
+def _require(monkeypatch: pytest.MonkeyPatch, environ: dict[str, str]) -> str:
+    # Replace the whole environ for the call so the PYTEST_* short-circuit in
+    # _require_env doesn't fire — we want the real production behaviour.
+    monkeypatch.setattr(envmod.os, "environ", dict(environ))
+    return envmod._require_env("DECNET_ADMIN_PASSWORD")
+
+
+def test_admin_password_unset_raises(monkeypatch: pytest.MonkeyPatch) -> None:
+    with pytest.raises(ValueError, match="not set"):
+        _require(monkeypatch, {})
+
+
+def test_admin_password_known_bad_default_raises(monkeypatch: pytest.MonkeyPatch) -> None:
+    with pytest.raises(ValueError, match="insecure default"):
+        _require(monkeypatch, {"DECNET_ADMIN_PASSWORD": "admin"})
+
+
+@pytest.mark.parametrize("bad", ["secret", "password", "changeme", "ADMIN"])
+def test_admin_password_other_known_bad_raises(monkeypatch: pytest.MonkeyPatch, bad: str) -> None:
+    with pytest.raises(ValueError, match="insecure default"):
+        _require(monkeypatch, {"DECNET_ADMIN_PASSWORD": bad})
+
+
+def test_admin_password_too_short_raises_in_production(monkeypatch: pytest.MonkeyPatch) -> None:
+    with pytest.raises(ValueError, match="too short"):
+        _require(monkeypatch, {"DECNET_ADMIN_PASSWORD": "short1"})
+
+
+def test_admin_password_short_allowed_in_developer_mode(monkeypatch: pytest.MonkeyPatch) -> None:
+    val = _require(monkeypatch, {"DECNET_ADMIN_PASSWORD": "short1", "DECNET_DEVELOPER": "true"})
+    assert val == "short1"
+
+
+def test_admin_password_strong_value_passes(monkeypatch: pytest.MonkeyPatch) -> None:
+    val = _require(monkeypatch, {"DECNET_ADMIN_PASSWORD": "a-strong-unique-password"})
+    assert val == "a-strong-unique-password"
+
+
+def test_pytest_short_circuit_returns_value_unchecked() -> None:
+    # Under live pytest (PYTEST_* present), _require_env returns the configured
+    # value without the production checks — documents why the dev loop is safe.
+    # conftest sets a strong value, so this also proves lazy resolution works.
+    assert envmod._require_env("DECNET_ADMIN_PASSWORD") == "test-password-123"
+
+
+def test_lazy_getattr_resolves_admin_password() -> None:
+    # Accessing the attribute (not a module global anymore) routes through
+    # __getattr__ -> _require_env.
+    assert envmod.DECNET_ADMIN_PASSWORD == "test-password-123"
+    with pytest.raises(AttributeError):
+        envmod.NOT_A_REAL_SECRET  # noqa: B018