feat(helpdesk): add customer engagement analytics page [skip ci]

New "Customer analytics" page under Monitoring: surfaces who is actively in
contact vs. who has gone quiet, so the team can reach out before a customer
slips away.

- KPI tiles: total / active / no-contact / never-in-contact
- "Customers without recent contact" — longest silence first
- "Top customers by communication" — most ticket activity in the period
- Configurable no-contact threshold (helpdesk settings, default 60 days)

CustomerEngagementService aggregates the global ticket snapshot per customer
(same single-pull model as RiskRadarService — no N+1 to BC). Last contact is
the most recent ticket activity; silence beyond the threshold flags the
customer. Revenue ranking deferred (BC OData exposes no per-customer ledger).

All within modules/helpdesk/. Tests + PHPStan green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

modules/helpdesk/web/css/helpdesk.css

@@ -3133,4 +3133,77 @@
     vertical-align: middle;
     color: var(--app-muted, #6c757d);
   }
+
+  /* --- Customer analytics --- */
+  .helpdesk-analytics-toolbar {
+    display: flex;
+    flex-wrap: wrap;
+    align-items: center;
+    justify-content: space-between;
+    gap: calc(var(--app-spacing) * 0.5);
+    margin-bottom: calc(var(--app-spacing) * 0.75);
+  }
+
+  .helpdesk-analytics-threshold-note {
+    margin: 0;
+    font-size: var(--text-sm, 0.875rem);
+    color: var(--app-muted, #6c757d);
+  }
+
+  /* Stat tiles here are read-only (rendered as <div>), so suppress the link affordances. */
+  .helpdesk-analytics-tile {
+    cursor: default;
+  }
+
+  .helpdesk-analytics-tile:hover {
+    box-shadow: none;
+  }
+
+  .helpdesk-analytics-block {
+    margin-top: calc(var(--app-spacing) * 1.5);
+  }
+
+  .helpdesk-analytics-block-header {
+    margin-bottom: calc(var(--app-spacing) * 0.5);
+  }
+
+  .helpdesk-analytics-block-header h2 {
+    margin: 0;
+    font-size: var(--text-lg, 1.125rem);
+  }
+
+  .helpdesk-analytics-block-header .muted {
+    margin: 0.15rem 0 0;
+    font-size: var(--text-sm, 0.875rem);
+    color: var(--app-muted, #6c757d);
+  }
+
+  .helpdesk-analytics-table {
+    width: 100%;
+    border-collapse: collapse;
+  }
+
+  .helpdesk-analytics-table th,
+  .helpdesk-analytics-table td {
+    padding: 0.5rem 0.75rem;
+    border-bottom: 1px solid var(--app-border, #dee2e6);
+    text-align: left;
+    vertical-align: middle;
+  }
+
+  .helpdesk-analytics-table th.num,
+  .helpdesk-analytics-table td.num {
+    text-align: right;
+    white-space: nowrap;
+  }
+
+  .helpdesk-analytics-cust-no {
+    color: var(--app-muted, #6c757d);
+    font-size: var(--text-sm, 0.875rem);
+  }
+
+  .helpdesk-analytics-never {
+    color: var(--app-muted, #6c757d);
+    font-style: italic;
+  }
 }