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>

src/Meezi.Core/Entities/SubscriptionPayment.cs

@@ -13,5 +13,13 @@ public class SubscriptionPayment : TenantEntity
     public string? RefId { get; set; }
     public SubscriptionPaymentStatus Status { get; set; } = SubscriptionPaymentStatus.Pending;
 
+    /// <summary>When this paid period starts. For an immediately-activated purchase this is
+    /// (around) the payment time; for a queued (Scheduled) purchase it is the end of the
+    /// current coverage. Null until the payment completes.</summary>
+    public DateTime? EffectiveFrom { get; set; }
+
+    /// <summary>When this paid period ends (EffectiveFrom + Months). Null until completed.</summary>
+    public DateTime? EffectiveTo { get; set; }
+
     public Cafe Cafe { get; set; } = null!;
 }

src/Meezi.Core/Enums/SubscriptionPaymentStatus.cs

@@ -4,5 +4,9 @@ public enum SubscriptionPaymentStatus
 {
     Pending = 0,
     Completed = 1,
-    Failed = 2
+    Failed = 2,
+    /// <summary>Paid, but queued to start after the current coverage ends.</summary>
+    Scheduled = 3,
+    /// <summary>A queued (Scheduled) subscription the owner cancelled before it started.</summary>
+    Cancelled = 4
 }