feat(reminders): SMS channel for reminder jobs

Tenants paying for the SMS add-on still got email-only reminders —
the channel column existed but nothing armed or dispatched sms jobs.
Booking creation now adds a second job when SMS is enabled and the
customer left a phone; dispatch routes sms jobs through an injected
sender adapter wired to the existing sms.Service, so monthly limits,
usage metering, and Stripe billing apply unchanged. Lifecycle handling
from the previous milestone clears and re-arms both channels together.

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/internal/notifications/service_test.go

@@ -138,3 +138,50 @@ func TestDispatchDueSkipsNonConfirmedBooking(t *testing.T) {
 		t.Fatalf("expected 1 skipped, got %+v", response)
 	}
 }
+
+func TestDispatchDueSendsSmsReminder(t *testing.T) {
+	repo := db.NewMemoryRepository()
+	startsAt := time.Now().UTC().Add(26 * time.Hour)
+	created, err := repo.CreateBooking(context.Background(), db.CreateBookingParams{
+		TenantID:      "5d6b3551-0a3e-4b86-bdf0-e9df20a47148",
+		BookingMode:   "appointment",
+		CustomerPhone: "+420777000111",
+		StartsAt:      startsAt,
+		EndsAt:        startsAt.Add(time.Hour),
+		Status:        "confirmed",
+		Reference:     "BK-TEST-SMS",
+	})
+	if err != nil {
+		t.Fatalf("create booking: %v", err)
+	}
+	if err := repo.CreateReminderJob(context.Background(), db.ReminderJobParams{
+		TenantID:     "5d6b3551-0a3e-4b86-bdf0-e9df20a47148",
+		BookingID:    created.ID,
+		Channel:      "sms",
+		ScheduledFor: time.Now().UTC().Add(-time.Minute),
+	}); err != nil {
+		t.Fatalf("create reminder job: %v", err)
+	}
+
+	var sentPhone, sentBody string
+	service := NewService(config.Config{Environment: "development"}, repo)
+	service.SetSMSSender(func(_ context.Context, tenantID, phone, body string) error {
+		sentPhone = phone
+		sentBody = body
+		return nil
+	})
+
+	response, err := service.DispatchDue(context.Background(), 10)
+	if err != nil {
+		t.Fatalf("dispatch due: %v", err)
+	}
+	if response.SentCount != 1 {
+		t.Fatalf("expected 1 sent, got %+v", response)
+	}
+	if sentPhone != "+420777000111" {
+		t.Fatalf("expected phone recipient, got %q", sentPhone)
+	}
+	if sentBody == "" {
+		t.Fatal("expected rendered sms body")
+	}
+}