fix(security): prevent removing the last active admin

Admins could remove the admin group from, deactivate, or delete the last
active admin, locking the system out of all administration. Add a
count_active_admins() repo method and a _is_last_active_admin() guard, and
block all three operations when they would leave zero active admins.

Refs: porchlight-yq7

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_admin/test_admin_routes.py

@@ -463,3 +463,70 @@ async def test_admin_cannot_delete_users_last_credential(client: AsyncClient) ->
     # The last credential must survive, and the admin is told why.
     assert await cred_repo.get_password_by_user(target.userid) is not None
     assert "last credential" in res.text.lower()
+
+
+async def _make_sole_admin_target(client: AsyncClient) -> User:
+    """Create a second admin, then return it as the target. The logged-in
+    admin from _login plus this one means 2 admins; callers deactivate the
+    _login admin first if they need this to be the *last* admin."""
+    return await _create_target_user(client, userid="admin2-01", username="admin2", groups=["admin"])
+
+
+@pytest.mark.asyncio
+async def test_cannot_deactivate_last_active_admin(client: AsyncClient) -> None:
+    await _login(client)  # the only admin
+    app = client._transport.app  # type: ignore[union-attr]
+    admin = await app.state.user_repo.get_by_username("admin")
+
+    token = await get_csrf_token(client)
+    res = await client.post(
+        f"/admin/users/{admin.userid}/deactivate",
+        headers={"X-CSRF-Token": token, "HX-Request": "true"},
+    )
+    still = await app.state.user_repo.get_by_userid(admin.userid)
+    assert still.active is True
+    msg = res.text.lower()
+    assert "last" in msg
+    assert "admin" in msg
+
+
+@pytest.mark.asyncio
+async def test_cannot_remove_admin_group_from_last_admin(client: AsyncClient) -> None:
+    await _login(client)
+    app = client._transport.app  # type: ignore[union-attr]
+    admin = await app.state.user_repo.get_by_username("admin")
+
+    token = await get_csrf_token(client)
+    res = await client.post(
+        f"/admin/users/{admin.userid}/groups",
+        data={"groups": "users"},  # drops "admin"
+        headers={"X-CSRF-Token": token, "HX-Request": "true"},
+    )
+    still = await app.state.user_repo.get_by_userid(admin.userid)
+    assert "admin" in still.groups
+    msg = res.text.lower()
+    assert "last" in msg
+    assert "admin" in msg
+
+
+@pytest.mark.asyncio
+async def test_cannot_delete_last_active_admin(client: AsyncClient) -> None:
+    await _login(client)
+    app = client._transport.app  # type: ignore[union-attr]
+    # A second, separate admin to delete (not self — self-delete is blocked separately).
+    target = await _create_target_user(client, userid="otheradmin-01", username="otheradmin", groups=["admin"])
+    # Deactivate the logged-in admin's peer? Instead make target the only ACTIVE admin
+    # by deactivating the _login admin via repo, so deleting target would orphan admins.
+    login_admin = await app.state.user_repo.get_by_username("admin")
+    await app.state.user_repo.update(login_admin.model_copy(update={"active": False}))
+
+    token = await get_csrf_token(client)
+    res = await client.request(
+        "DELETE",
+        f"/admin/users/{target.userid}",
+        headers={"X-CSRF-Token": token, "HX-Request": "true"},
+    )
+    assert await app.state.user_repo.get_by_userid(target.userid) is not None
+    msg = res.text.lower()
+    assert "last" in msg
+    assert "admin" in msg