feat: add incremental log scanning with resumable byte offsets

Track last processed byte offset per log file so scans only read new
entries, add --full flag to force a rescan, and persist state atomically.

app/templates/dashboard.html

@@ -96,5 +96,7 @@ endblock %} {% block content %}
   }
 
   loadDashboard();
+  // Keep the latest-scans table fresh without hammering /api/scan/status.
+  setInterval(loadDashboard, 15000);
 </script>
 {% endblock %}