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.Infrastructure/Data/Migrations/20260602130649_AddSubscriptionScheduling.cs

@@ -0,0 +1,39 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Meezi.Infrastructure.Data.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddSubscriptionScheduling : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<DateTime>(
+                name: "EffectiveFrom",
+                table: "SubscriptionPayments",
+                type: "timestamp with time zone",
+                nullable: true);
+
+            migrationBuilder.AddColumn<DateTime>(
+                name: "EffectiveTo",
+                table: "SubscriptionPayments",
+                type: "timestamp with time zone",
+                nullable: true);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "EffectiveFrom",
+                table: "SubscriptionPayments");
+
+            migrationBuilder.DropColumn(
+                name: "EffectiveTo",
+                table: "SubscriptionPayments");
+        }
+    }
+}

src/Meezi.Infrastructure/Data/Migrations/AppDbContextModelSnapshot.cs

@@ -2011,6 +2011,12 @@ namespace Meezi.Infrastructure.Data.Migrations
                     b.Property<DateTime?>("DeletedAt")
                         .HasColumnType("timestamp with time zone");
 
+                    b.Property<DateTime?>("EffectiveFrom")
+                        .HasColumnType("timestamp with time zone");
+
+                    b.Property<DateTime?>("EffectiveTo")
+                        .HasColumnType("timestamp with time zone");
+
                     b.Property<int>("Months")
                         .HasColumnType("integer");