feat(billing): queue subscriptions bought while active + cancel queued

Before, buying a plan immediately switched the tier and stacked the duration.
Now a purchase made while the café still has paid coverage is QUEUED to start
when the current coverage ends, and the owner can cancel a queued one.

Model:
- SubscriptionPayment gains EffectiveFrom/EffectiveTo; status gains Scheduled
  (paid, queued) and Cancelled. EF migration AddSubscriptionScheduling (nullable).

BillingService:
- On payment completion, compute coverage end (latest of active expiry + furthest
  queued period). If it is in the future → Scheduled (queued, café tier/expiry
  untouched); else activate immediately as before. Periods chain correctly.
- GetStatusAsync lazily promotes any due queued period to active, and returns the
  queue (QueuedPlans).
- CancelQueuedAsync cancels a Scheduled period (owner-only) and re-packs the queue
  so later periods slide earlier. Active prepaid plan is never cut short; no
  automatic refund (manual, per product decision).
- Confirmation SMS distinguishes "activated until X" vs "queued, starts X".

API: BillingStatusDto.QueuedPlans + DELETE /api/billing/queued/{paymentId}.

Dashboard:
- Subscription screen shows a "Queued subscriptions" card (tier, window, cancel
  with confirm).
- Checkout shows "you already have an active subscription — this will start on
  {date}" when the café is still covered.
- i18n fa/en/ar.

81 API tests pass; dashboard typechecks.

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

web/dashboard/messages/ar.json

@@ -1047,7 +1047,18 @@
       "secureNote": "تتم المعالجة عبر بوابة دفع بنكية آمنة.",
       "payTotal": "ادفع {total}",
       "redirecting": "جارٍ التحويل إلى البوابة...",
-      "paymentFailed": "فشل الدفع. الرجاء المحاولة مرة أخرى."
+      "paymentFailed": "فشل الدفع. الرجاء المحاولة مرة أخرى.",
+      "queuedNotice": "لديك اشتراك نشط بالفعل. ستتم إضافة هذا الشراء إلى قائمة الانتظار وسيبدأ في {date}."
+    },
+    "queued": {
+      "title": "الاشتراكات في قائمة الانتظار",
+      "subtitle": "تبدأ تلقائيًا عند انتهاء اشتراكك الحالي.",
+      "months": "{count} أشهر",
+      "window": "من {from} إلى {to}",
+      "cancel": "إلغاء",
+      "cancelled": "تم إلغاء الاشتراك في قائمة الانتظار",
+      "cancelConfirmTitle": "إلغاء الاشتراك المجدول",
+      "cancelConfirmDesc": "إلغاء اشتراك {plan} المقرر أن يبدأ في {from}؟ لن يتأثر اشتراكك الحالي."
     }
   },
   "settings": {