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>

server.js

@@ -418,6 +418,16 @@ app.get('/api/executions', authenticateSSO, async (req, res) => {
   }
 });
 
+app.delete('/api/executions/:id', authenticateSSO, async (req, res) => {
+  try {
+    await pool.query('DELETE FROM executions WHERE id = ?', [req.params.id]);
+    broadcast({ type: 'execution_deleted', execution_id: req.params.id });
+    res.json({ success: true });
+  } catch (error) {
+    res.status(500).json({ error: error.message });
+  }
+});
+
 // Health check (no auth required)
 app.get('/health', async (req, res) => {
   try {