Add queue high water marks to handler statistics

- Track the maximum queue length seen for each handler
- Display high water marks on the status page with percentage
- Helps identify which handlers are experiencing queue pressure

internal/templates/status.html

@@ -264,6 +264,10 @@
                         <span class="metric-label">Queue</span>
                         <span class="metric-value">${handler.queue_length}/${handler.queue_capacity}</span>
                     </div>
+                    <div class="metric">
+                        <span class="metric-label">High Water Mark</span>
+                        <span class="metric-value">${handler.queue_high_water_mark}/${handler.queue_capacity} (${Math.round(handler.queue_high_water_mark * 100 / handler.queue_capacity)}%)</span>
+                    </div>
                     <div class="metric">
                         <span class="metric-label">Processed</span>
                         <span class="metric-value">${formatNumber(handler.processed_count)}</span>