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/src/components/subscription/checkout-screen.tsx

@@ -68,6 +68,37 @@ export function CheckoutScreen() {
     enabled: !!cafeId && isCafeOwner(role),
   });
 
+  // If the owner is still covered (active plan and/or queued plans), this purchase will be
+  // queued to start when the current coverage ends rather than activating immediately.
+  const { data: billingStatus } = useQuery({
+    queryKey: ["billing-status", cafeId],
+    queryFn: () =>
+      apiGet<{
+        planTier: string;
+        planExpiresAt: string | null;
+        isPlanExpired: boolean;
+        queuedPlans: { effectiveTo: string }[];
+      }>("/api/billing/status"),
+    enabled: !!cafeId && isCafeOwner(role),
+  });
+
+  const coverageEnd = useMemo(() => {
+    if (!billingStatus) return null;
+    const now = Date.now();
+    let end = 0;
+    if (
+      billingStatus.planTier !== "Free" &&
+      billingStatus.planExpiresAt &&
+      !billingStatus.isPlanExpired
+    ) {
+      end = Math.max(end, new Date(billingStatus.planExpiresAt).getTime());
+    }
+    for (const q of billingStatus.queuedPlans ?? []) {
+      end = Math.max(end, new Date(q.effectiveTo).getTime());
+    }
+    return end > now ? new Date(end) : null;
+  }, [billingStatus]);
+
   useEffect(() => {
     if (!paymentMethod && paymentMethods.length > 0) {
       const def = paymentMethods.find((m) => m.isDefault) ?? paymentMethods[0];
@@ -140,6 +171,13 @@ export function CheckoutScreen() {
         }
       />
 
+      {coverageEnd ? (
+        <div className="flex items-start gap-2 rounded-xl border border-[#0F6E56]/30 bg-[#E1F5EE]/50 px-4 py-3 text-sm text-[#0F6E56]">
+          <ShieldCheck className="mt-0.5 h-4 w-4 shrink-0" aria-hidden />
+          <p>{tc("queuedNotice", { date: coverageEnd.toLocaleDateString(numberLocale) })}</p>
+        </div>
+      ) : null}
+
       {/* Factor / invoice */}
       <Card className="overflow-hidden rounded-xl border border-border/80 shadow-sm">
         {/* Invoice header */}