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.

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

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

apps/backend/openapi/bookra.openapi.yaml

@@ -396,6 +396,97 @@ paths:
           description: Invalid manage token
         "409":
           description: New slot conflicts or booking already cancelled
+  /v1/public/bookings/{reference}/sessions:
+    get:
+      tags: [Public Booking]
+      operationId: listSessionMoveTargets
+      summary: Upcoming sessions a class booking can move to
+      description: >-
+        Class bookings cannot reschedule to arbitrary windows — they move
+        between sessions of the same template. Returns upcoming same-template
+        sessions with live capacity info.
+      parameters:
+        - in: path
+          name: reference
+          required: true
+          schema:
+            type: string
+        - in: query
+          name: token
+          required: true
+          schema:
+            type: string
+      responses:
+        "200":
+          description: Move targets with capacity info
+          content:
+            application/json:
+              schema:
+                type: object
+                properties:
+                  sessions:
+                    type: array
+                    items:
+                      $ref: "#/components/schemas/SessionMoveTarget"
+        "400":
+          description: Not a live class booking
+        "403":
+          description: Invalid manage token
+        "404":
+          description: Booking not found
+        "409":
+          description: Booking already cancelled
+  /v1/public/bookings/{reference}/move:
+    post:
+      tags: [Public Booking]
+      operationId: moveBookingToSession
+      summary: Move a class booking to another session of the same template
+      description: >-
+        Retargets the booking to the target session. Confirmed bookings free
+        their old seat (draining its waitlist); waitlisted bookings release
+        their queue position. The booking is confirmed when the target has a
+        free seat, otherwise it joins the target's waitlist.
+      parameters:
+        - in: path
+          name: reference
+          required: true
+          schema:
+            type: string
+        - in: query
+          name: token
+          required: true
+          schema:
+            type: string
+      requestBody:
+        required: true
+        content:
+          application/json:
+            schema:
+              type: object
+              required: [classSessionId]
+              properties:
+                classSessionId:
+                  type: string
+                  format: uuid
+      responses:
+        "200":
+          description: Booking moved; status is the resulting booking state
+          content:
+            application/json:
+              schema:
+                type: object
+                properties:
+                  status:
+                    type: string
+                    enum: [confirmed, waitlisted]
+        "400":
+          description: Not a live class booking, or target is not an upcoming same-template session
+        "403":
+          description: Invalid manage token
+        "404":
+          description: Booking not found
+        "409":
+          description: Booking already cancelled
   /v1/public/bookings/{reference}/cancel:
     post:
       tags: [Public Booking]
@@ -1902,6 +1993,31 @@ components:
         waitlistPosition:
           type: integer
           description: 1-based queue spot for waitlisted class bookings; absent otherwise
+    SessionMoveTarget:
+      type: object
+      properties:
+        sessionId:
+          type: string
+          format: uuid
+        title:
+          type: string
+        startsAt:
+          type: string
+          format: date-time
+        endsAt:
+          type: string
+          format: date-time
+        location:
+          type: string
+        capacity:
+          type: integer
+        confirmedCount:
+          type: integer
+        waitlistCount:
+          type: integer
+        remaining:
+          type: integer
+          description: Free confirmed seats; 0 or negative means the move lands on the waitlist
     CalendarFeedEntry:
       type: object
       properties: