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/fa.json

@@ -1152,6 +1152,15 @@
         "save": "ذخیره",
         "saved": "ذخیره شد",
         "loading": "در حال بارگذاری..."
+      },
+      "grant": {
+        "title": "افزودن اشتراک رایگان",
+        "plan": "پلن",
+        "months": "تعداد ماه",
+        "submit": "اعطا",
+        "granted": "اشتراک اعطا شد",
+        "failed": "اعطای اشتراک ناموفق بود",
+        "currentExpiry": "انقضای فعلی"
       }
     },
     "integrations": {