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.API/Controllers/BillingController.cs

@@ -103,4 +103,25 @@ public class BillingController : ControllerBase
 
         return Ok(new ApiResponse<BillingStatusDto>(true, data));
     }
+
+    [Authorize]
+    [HttpDelete("api/billing/queued/{paymentId}")]
+    public async Task<IActionResult> CancelQueued(string paymentId, ITenantContext tenant, CancellationToken ct)
+    {
+        if (string.IsNullOrEmpty(tenant.CafeId))
+            return Unauthorized();
+        if (tenant.Role != Core.Enums.EmployeeRole.Owner)
+            return StatusCode(403, new ApiResponse<object>(false, null,
+                new ApiError("OWNER_REQUIRED", "Only the cafe owner can manage subscription billing.")));
+
+        var (ok, code, message) = await _billing.CancelQueuedAsync(tenant.CafeId, paymentId, ct);
+        if (!ok)
+        {
+            return code == "NOT_FOUND"
+                ? NotFound(new ApiResponse<object>(false, null, new ApiError(code, message ?? "Not found.")))
+                : BadRequest(new ApiResponse<object>(false, null, new ApiError(code ?? "ERROR", message ?? "Failed.")));
+        }
+
+        return Ok(new ApiResponse<object>(true, new { id = paymentId }));
+    }
 }