Function upsert_fallback_key 

pub fn upsert_fallback_key(
    conn: &Connection,
    user_id: i64,
    device_id: &str,
    algorithm: &str,
    key_id: &str,
    key_json: &str,
    now: &str,
) -> Result<()>

Upload (or replace) the one active fallback key for algorithm — replacing always resets used back to 0, per spec (“a new fallback key is unused until it is actually claimed”).