feat(bookings): dashboard cancellation and waitlist promotion

- POST /v1/catalog/bookings/{id}/cancel lets businesses cancel
  tenant-owned bookings; two-step confirm in the detail modal
- Shared PromoteWaitlist helper promotes earliest waitlisted bookings
  when a class seat frees, on both dashboard and customer cancel paths;
  promoted customers get a confirmation email, waitlist ledger marked
  resolved
- waitlisted status surfaced in bookings UI: stats card, filter, warning
  badge, demo seed, CS+EN strings
- BookingRecord gains created_at for fair ordering; GetBookingByID and
  ResolveWaitlistEntry added to the repository surface

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

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

apps/backend/internal/bookings/lifecycle.go

@@ -0,0 +1,64 @@
+package bookings
+
+import (
+	"context"
+	"errors"
+	"sort"
+	"time"
+
+	"github.com/jackc/pgx/v5"
+
+	"bookra/apps/backend/internal/db"
+)
+
+// PromoteWaitlist confirms the earliest waitlisted bookings on a class session
+// while free capacity remains. It returns the promoted records so callers can
+// notify customers. A missing session is treated as nothing to promote.
+func PromoteWaitlist(ctx context.Context, repo db.Repository, tenantID, sessionID string) ([]db.BookingRecord, error) {
+	session, err := repo.GetClassSessionByID(ctx, sessionID)
+	if err != nil {
+		if errors.Is(err, pgx.ErrNoRows) {
+			return nil, nil
+		}
+		return nil, err
+	}
+
+	bookings, err := repo.ListBookingsByTenantBetween(ctx, tenantID, session.StartsAt.Add(-time.Second), session.EndsAt.Add(time.Second))
+	if err != nil {
+		return nil, err
+	}
+
+	confirmed := 0
+	var waitlisted []db.BookingRecord
+	for _, booking := range bookings {
+		if booking.ClassSessionID == nil || *booking.ClassSessionID != sessionID {
+			continue
+		}
+		switch booking.Status {
+		case "confirmed":
+			confirmed++
+		case "waitlisted":
+			waitlisted = append(waitlisted, booking)
+		}
+	}
+	if len(waitlisted) == 0 {
+		return nil, nil
+	}
+	sort.SliceStable(waitlisted, func(i, j int) bool {
+		return waitlisted[i].CreatedAt.Before(waitlisted[j].CreatedAt)
+	})
+
+	var promoted []db.BookingRecord
+	for _, booking := range waitlisted {
+		if session.Capacity > 0 && confirmed >= int(session.Capacity) {
+			break
+		}
+		if err := repo.UpdateBookingStatus(ctx, booking.ID, "confirmed"); err != nil {
+			return promoted, err
+		}
+		_ = repo.ResolveWaitlistEntry(ctx, tenantID, sessionID, booking.CustomerEmail)
+		confirmed++
+		promoted = append(promoted, booking)
+	}
+	return promoted, nil
+}