Implement WHMCS Addon Dashboard with API for stats and integrate billing provider logic

docs/integrations/whmcs/addon-module.md

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+# WHMCS addon module
+
+The addon provides global GameCloud administration inside WHMCS. **WHMCS remains the commercial system of record**; this module does not process payments.
+
+## Install
+
+1. Copy `integrations/whmcs/modules/addons/hexagamecloud` to `/modules/addons/hexagamecloud`
+2. **Setup → Addon Modules** → activate **HexaHost GameCloud**
+3. Configure:
+   - **GameCloud API URL** — e.g. `https://api.example.net`
+   - **Integration ID** — matches `WHMCS_INTEGRATION_ID` on GameCloud
+   - **API Secret** — matches `WHMCS_API_SECRET`
+4. Copy `integrations/whmcs/hooks/hexagamecloud.php` to WHMCS `includes/hooks/hexagamecloud.php`
+
+## Pages
+
+| Tab | Purpose |
+|-----|---------|
+| Dashboard | Health, counts, last sync |
+| Events | Poll, view, ack, dead-letter |
+| Reconciliation | Dry-run / live reconcile |
+
+## Cron
+
+The daily hook polls pending GameCloud events and optionally runs a dry reconciliation when **Auto-reconcile on daily cron** is enabled.
+
+## Billing
+
+Set `BILLING_PROVIDER=whmcs` on GameCloud in production. Stripe (`BILLING_PROVIDER=stripe`) is an optional fallback for standalone deployments without WHMCS — see [billing providers](../../billing/providers.md).