Make Non-Compliant stat clickable — reveals metric breakdown buttons

Clicking the Non-Compliant card on the CCP Metrics overview now toggles a
panel of metric buttons below it, each showing the metric ID, category,
non-compliant count, and compliance % vs target. Styled like the compliance
page's MetricHealthCard pattern.

Backend: added metric_breakdown to the /stats response — aggregated
cross-vertical metric totals (ALL: rows only, grouped by metric_id).

Also updated tech steering file to document the single-port Express
architecture and the requirement to run npm run build after frontend changes.

backend/routes/vclMultiVertical.js

@@ -436,6 +436,14 @@ function createVCLMultiVerticalRouter(upload) {
      *       forecast_burndown: Array<{ month: string, projected_remaining: number }>,
      *       last_upload: string|null
      *     }>,
+     *     metric_breakdown: Array<{
+     *       metric_id: string,
+     *       category: string,
+     *       non_compliant: number,
+     *       compliant: number,
+     *       total: number,
+     *       target: number
+     *     }>,
      *     last_upload_date: string|null
      *   }
      * @response 500 { error: string }
@@ -540,6 +548,19 @@ function createVCLMultiVerticalRouter(upload) {
                 };
             });
 
+            // Cross-vertical metric breakdown (aggregated across all verticals, ALL: rows only)
+            const { rows: metricBreakdown } = await pool.query(`
+                SELECT metric_id, category,
+                       SUM(non_compliant)::int AS non_compliant,
+                       SUM(compliant)::int AS compliant,
+                       SUM(total)::int AS total,
+                       ROUND(AVG(target::numeric), 2) AS target
+                FROM vcl_multi_vertical_summary
+                WHERE upload_id = ANY($1) AND team LIKE 'ALL:%'
+                GROUP BY metric_id, category
+                ORDER BY non_compliant DESC
+            `, [latestUploadIds]);
+
             res.json({
                 stats: {
                     total_devices: aggTotal,
@@ -550,6 +571,7 @@ function createVCLMultiVerticalRouter(upload) {
                 },
                 donut,
                 vertical_breakdown,
+                metric_breakdown: metricBreakdown,
                 last_upload_date: uploadDates.length > 0 ? uploadDates.reduce((max, r) => r.last_upload > max ? r.last_upload : max, '') : null,
             });
         } catch (err) {