feat(admin): grant a free subscription to any café from the admin panel

Adds POST /api/admin/cafes/{cafeId}/grant-subscription (admin-auth): sets the
café's plan and adds N months of coverage, appended to any time it already has so
a grant never shortens existing paid time. Records the gift as a SubscriptionPayment
(provider Manual, amount 0, Completed) for billing history/audit. New
PaymentProvider.Manual = 4 (int append, no migration). Admin-web café cards get a
"grant free subscription" panel (plan select + months + apply), showing the current
expiry; fa/en/ar strings.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/admin/messages/ar.json

@@ -1159,6 +1159,15 @@
         "save": "حفظ",
         "saved": "تم الحفظ",
         "loading": "جاري التحميل..."
+      },
+      "grant": {
+        "title": "منح اشتراك مجاني",
+        "plan": "الباقة",
+        "months": "عدد الأشهر",
+        "submit": "منح",
+        "granted": "تم منح الاشتراك",
+        "failed": "تعذّر منح الاشتراك",
+        "currentExpiry": "انتهاء الصلاحية الحالي"
       }
     },
     "integrations": {