feat(bookings): session-to-session move for class bookings

Class customers can move a booking to another future session of the
same template from the manage page. Full target sessions place the
booking on the waitlist; freeing a confirmed seat drains the old
session's queue via the existing promotion path. Reminders re-arm
for the new time.

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

apps/backend/internal/db/repository.go

@@ -112,6 +112,7 @@ type Repository interface {
 	GetWaitlistPosition(ctx context.Context, sessionID, bookingID string) (int, error)
 	UpdateBookingStatus(ctx context.Context, bookingID string, status string) error
 	RescheduleBooking(ctx context.Context, bookingID string, startsAt time.Time, endsAt time.Time) error
+	UpdateBookingSession(ctx context.Context, bookingID string, sessionID string, startsAt time.Time, endsAt time.Time, status string) error
 
 	// Working Hours
 	ListWorkingHoursByTenant(ctx context.Context, tenantID string, staffID *string) ([]WorkingHoursRecord, error)
@@ -780,6 +781,15 @@ func (r *PGRepository) RescheduleBooking(ctx context.Context, bookingID string,
 	return err
 }
 
+// UpdateBookingSession retargets a class booking to a different session and
+// carries the session's own time window onto the booking.
+func (r *PGRepository) UpdateBookingSession(ctx context.Context, bookingID string, sessionID string, startsAt time.Time, endsAt time.Time, status string) error {
+	_, err := r.pool.Exec(ctx, `
+		UPDATE bookings SET class_session_id = $2, starts_at = $3, ends_at = $4, status = $5, updated_at = now() WHERE id = $1
+	`, bookingID, sessionID, startsAt, endsAt, status)
+	return err
+}
+
 // ============================================
 // WORKING HOURS METHODS - PG REPOSITORY (STUBS)
 // ============================================
@@ -2002,6 +2012,19 @@ func (r *MemoryRepository) RescheduleBooking(_ context.Context, bookingID string
 	return pgx.ErrNoRows
 }
 
+func (r *MemoryRepository) UpdateBookingSession(_ context.Context, bookingID string, sessionID string, startsAt time.Time, endsAt time.Time, status string) error {
+	for i, b := range r.bookings {
+		if b.ID == bookingID {
+			r.bookings[i].ClassSessionID = &sessionID
+			r.bookings[i].StartsAt = startsAt
+			r.bookings[i].EndsAt = endsAt
+			r.bookings[i].Status = status
+			return nil
+		}
+	}
+	return pgx.ErrNoRows
+}
+
 // ============================================
 // EMAIL TEMPLATE METHODS - MEMORY REPOSITORY
 // ============================================