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/app/main.py

@@ -13,6 +13,7 @@ from app.auth.api_key import hash_api_key
 from app.config import get_settings
 from app.database import SessionLocal
 from app.models import ApiKey
+from app.services.user_auth import bootstrap_admin_user
 from app.version import APP_VERSION, APP_VERSION_LABEL
 
 ROOT = Path(__file__).resolve().parents[2]
@@ -43,10 +44,19 @@ def bootstrap_api_key() -> None:
         db.close()
 
 
+def bootstrap_users() -> None:
+    db = SessionLocal()
+    try:
+        bootstrap_admin_user(db)
+    finally:
+        db.close()
+
+
 @asynccontextmanager
 async def lifespan(_app: FastAPI):
     logger.info("%s — application startup (version %s)", APP_VERSION_LABEL, APP_VERSION)
     bootstrap_api_key()
+    bootstrap_users()
     yield
     logger.info("%s — application shutdown", APP_VERSION_LABEL)