feat(billing): renewals, coupons, VAT numbering, billing cycles and invoice payment tracking

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

app/Support/VatCalculator.php

@@ -0,0 +1,44 @@
+<?php
+
+namespace App\Support;
+
+use App\Models\Invoice;
+
+class VatCalculator
+{
+    public static function refresh(Invoice $invoice): Invoice
+    {
+        if ($invoice->vat_number) {
+            return $invoice;
+        }
+
+        $tax = self::taxFor($invoice);
+
+        if ((int) $invoice->tax !== $tax) {
+            $invoice->forceFill(['tax' => $tax])->saveQuietly();
+        }
+
+        return $invoice;
+    }
+
+    public static function taxFor(Invoice $invoice): int
+    {
+        $items = $invoice->invoiceItems()->with('product:id,product_type')->get();
+        $gross = (int) $items->sum('total_price');
+
+        if ($gross <= 0) {
+            return 0;
+        }
+
+        $payable = min((int) $invoice->total, $gross);
+        $tax = 0.0;
+
+        foreach ($items as $item) {
+            $rate = VatSettings::rateFor($item->product?->product_type);
+            $share = $payable * ((int) $item->total_price / $gross);
+            $tax += $share * $rate / (100 + $rate);
+        }
+
+        return (int) round($tax);
+    }
+}