feat: multi-user UI login with admin and monitor roles

Add sac_users table, JWT roles, settings/users API guarded for admin, Users page in UI, and sac_manage_user.py CLI.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/tests/test_users_auth.py

@@ -0,0 +1,64 @@
+"""Multi-user UI auth and RBAC."""
+
+from app.models.user import USER_ROLE_MONITOR
+
+
+def test_login_success(client):
+    response = client.post(
+        "/api/v1/auth/login",
+        json={"username": "test-admin", "password": "test-admin-password"},
+    )
+    assert response.status_code == 200
+    body = response.json()
+    assert body["username"] == "test-admin"
+    assert body["role"] == "admin"
+    assert body["access_token"]
+
+
+def test_login_monitor(client):
+    response = client.post(
+        "/api/v1/auth/login",
+        json={"username": "test-monitor", "password": "test-monitor-password"},
+    )
+    assert response.status_code == 200
+    assert response.json()["role"] == USER_ROLE_MONITOR
+
+
+def test_login_invalid_password(client):
+    response = client.post(
+        "/api/v1/auth/login",
+        json={"username": "test-admin", "password": "wrong"},
+    )
+    assert response.status_code == 401
+
+
+def test_auth_me(client, jwt_headers):
+    response = client.get("/api/v1/auth/me", headers=jwt_headers)
+    assert response.status_code == 200
+    assert response.json() == {"username": "test-admin", "role": "admin"}
+
+
+def test_settings_forbidden_for_monitor(client, jwt_monitor_headers):
+    response = client.get("/api/v1/settings/notifications", headers=jwt_monitor_headers)
+    assert response.status_code == 403
+
+
+def test_events_allowed_for_monitor(client, jwt_monitor_headers):
+    response = client.get("/api/v1/events", headers=jwt_monitor_headers)
+    assert response.status_code == 200
+
+
+def test_admin_create_monitor_user(client, jwt_headers):
+    response = client.post(
+        "/api/v1/users",
+        headers=jwt_headers,
+        json={"username": "new-monitor", "password": "longpassword1", "role": "monitor"},
+    )
+    assert response.status_code == 201
+    assert response.json()["username"] == "new-monitor"
+    assert response.json()["role"] == "monitor"
+
+
+def test_monitor_cannot_list_users(client, jwt_monitor_headers):
+    response = client.get("/api/v1/users", headers=jwt_monitor_headers)
+    assert response.status_code == 403