✨ feat: add update_check_interval_hours config field

Adds `update_check_interval_hours: u64` (serde default 24, 0 = disabled)
to Config struct, enabling periodic update check control via config file.

tests/scan.rs

@@ -10,6 +10,7 @@ fn make_config(max_depth: usize) -> Config {
         max_depth,
         cache_enabled: true,
         cache_ttl_hours: 24,
+        update_check_interval_hours: 24,
         default_session: SessionConfig { windows: vec![] },
     }
 }