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/domain/models.go

@@ -512,6 +512,29 @@ type RescheduleBookingRequest struct {
 	Reason      string `json:"reason,omitempty"`
 }
 
+// SessionMoveTarget is an upcoming session of the same class template a
+// customer can move their booking to. Remaining is the free confirmed seats;
+// negative means a queue would form.
+type SessionMoveTarget struct {
+	SessionID      string    `json:"sessionId"`
+	Title          string    `json:"title"`
+	StartsAt       time.Time `json:"startsAt"`
+	EndsAt         time.Time `json:"endsAt"`
+	Location       string    `json:"location,omitempty"`
+	Capacity       int32     `json:"capacity"`
+	ConfirmedCount int       `json:"confirmedCount"`
+	WaitlistCount  int       `json:"waitlistCount"`
+	Remaining      int       `json:"remaining"`
+}
+
+type MoveBookingRequest struct {
+	ClassSessionID string `json:"classSessionId" binding:"required"`
+}
+
+type MoveBookingResponse struct {
+	Status string `json:"status"` // confirmed | waitlisted
+}
+
 type CancelBookingRequest struct {
 	Reason string `json:"reason,omitempty"`
 }