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/lib/Module/Helpdesk/Service/DebitorCacheControl.php

@@ -97,6 +97,11 @@ final class DebitorCacheControl
         return 'module.helpdesk.risk-radar.v1.' . $tenantScope . '.' . $periodDays;
     }
 
+    public static function engagementKey(string $tenantScope, int $periodDays): string
+    {
+        return 'module.helpdesk.engagement.v1.' . $tenantScope . '.' . $periodDays;
+    }
+
     /** @api Used by BcODataGateway::fetchUpdateTickets() */
     public static function updateTicketsKey(string $tenantScope): string
     {