test: make reschedule reminder test weekday-agnostic

The test rescheduled a booking with a blind +48h shift. With Mon-Fri
availability rules the target lands on a weekend roughly a third of the
time (booking ~72h out + 48h), so it flaked by day of week. It now picks
a real availability slot >25h out, mirroring the manage-page picker and
keeping the -24h reminder tier deterministic.

Authored By: TDvorak <[email protected]>

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

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

apps/backend/internal/bookings/service_test.go

@@ -607,8 +607,40 @@ func TestRescheduleRearmsReminder(t *testing.T) {
 	resp := bookAppointmentSlot(t, repo, service)
 	booking, _ := repo.GetBookingByID(ctx, resp.BookingID)
 
-	newStartsAt := booking.StartsAt.Add(48 * time.Hour)
-	newEndsAt := booking.EndsAt.Add(48 * time.Hour)
+	// Reschedule into a real availability slot on a later day, mirroring the
+	// manage-page slot picker — a blind +48h shift can land on a weekend or
+	// blocked day and fail validation for the wrong reason.
+	var newStartsAt, newEndsAt time.Time
+	horizon := time.Now().UTC().Add(25 * time.Hour)
+	for offset := 1; offset <= 14 && newStartsAt.IsZero(); offset++ {
+		date := time.Now().UTC().AddDate(0, 0, offset).Format("2006-01-02")
+		availability, err := service.Availability(ctx, "studio-atelier", AvailabilityFilter{Date: date})
+		if err != nil {
+			t.Fatalf("availability for %s: %v", date, err)
+		}
+		for _, slot := range availability.Slots {
+			if slot.Mode != "appointment" {
+				continue
+			}
+			starts, err := time.Parse(time.RFC3339, slot.StartsAt)
+			if err != nil {
+				t.Fatalf("parse slot start: %v", err)
+			}
+			ends, err := time.Parse(time.RFC3339, slot.EndsAt)
+			if err != nil {
+				t.Fatalf("parse slot end: %v", err)
+			}
+			// Stay above the 25h tier so the reminder re-arms at -24h.
+			if starts.Equal(booking.StartsAt) || !starts.After(horizon) {
+				continue
+			}
+			newStartsAt, newEndsAt = starts, ends
+			break
+		}
+	}
+	if newStartsAt.IsZero() {
+		t.Fatal("no alternative appointment slot found within 14 days")
+	}
 	manageURL, _ := url.Parse(resp.ManageURL)
 	token := manageURL.Query().Get("token")
 	if err := customerService.RescheduleBooking(ctx, resp.Reference, domain.RescheduleBookingRequest{