Add execution cleanup functionality

Changes:
- Added DELETE /api/executions/:id endpoint
- Added "Clear Completed" button to Executions tab
- Deletes all completed and failed executions
- Broadcasts execution_deleted event to update all clients
- Shows count of deleted executions

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

public/index.html

@@ -633,7 +633,8 @@
         <div id="executions" class="tab-content">
             <div class="card">
                 <h3>Execution History</h3>
-                <button onclick="refreshData()">🔄 Refresh</button>
+                <button onclick="refreshData()">[ 🔄 Refresh ]</button>
+                <button onclick="clearCompletedExecutions()" style="margin-left: 10px;">[ 🗑️ Clear Completed ]</button>
                 <div id="executionList"><div class="loading">Loading...</div></div>
             </div>
         </div>
@@ -834,16 +835,42 @@
             }
         }
 
+        async function clearCompletedExecutions() {
+            if (!confirm('Delete all completed and failed executions?')) return;
+
+            try {
+                const response = await fetch('/api/executions');
+                const executions = await response.json();
+
+                const toDelete = executions.filter(e => e.status === 'completed' || e.status === 'failed');
+
+                if (toDelete.length === 0) {
+                    alert('No completed or failed executions to delete');
+                    return;
+                }
+
+                for (const execution of toDelete) {
+                    await fetch(`/api/executions/${execution.id}`, { method: 'DELETE' });
+                }
+
+                alert(`Deleted ${toDelete.length} execution(s)`);
+                refreshData();
+            } catch (error) {
+                console.error('Error clearing executions:', error);
+                alert('Error clearing executions');
+            }
+        }
+
         async function executeWorkflow(workflowId, name) {
             if (!confirm(`Execute workflow: ${name}?`)) return;
-            
+
             try {
                 const response = await fetch('/api/executions', {
                     method: 'POST',
                     headers: { 'Content-Type': 'application/json' },
                     body: JSON.stringify({ workflow_id: workflowId })
                 });
-                
+
                 if (response.ok) {
                     const data = await response.json();
                     alert('Workflow execution started!');