fix(reservations): a booking read on its own carries its day slots, so an edit no longer drops them

The create and update answers, the reservation:updated broadcast and the MCP
reservation tools all hand back the single booking read, and that read never
loaded reservation_day_positions. The day list puts a booking moved on a day by
that slot before its time, and the client replaces the booking with the answer,
so a booking moved on a day jumped back to its time after every edit until the
next reload. The read now attaches day_positions in the shape the trip list
already sends.

Refs #2631

server/src/db/repositories/ReservationDayPositions.repository.ts

@@ -32,6 +32,20 @@ export class ReservationDayPositionsRepository extends TrekRepository<Reservatio
       .execute<{ reservation_id: number; day_id: number; position: number }[]>('all', false);
   }
 
+  /**
+   * `SELECT day_id, position FROM reservation_day_positions WHERE reservation_id = ?`:
+   * one booking's slots, for the single-booking read the create, update and
+   * broadcast payloads come from. Without them a booking moved on a day lost
+   * that slot in the list after every edit until the next reload (#2631).
+   */
+  async listForReservation(reservation_id: number): Promise<{ day_id: number; position: number }[]> {
+    return this.kysely<ReservationDayPositionsKyselyDB>()
+      .selectFrom('reservation_day_positions')
+      .select(['day_id', 'position'])
+      .where('reservation_id', '=', reservation_id)
+      .execute();
+  }
+
   /**
    * RS31 (`ReservationsService.updatePositions`'s day branch) — `INSERT OR
    * REPLACE INTO reservation_day_positions (reservation_id, day_id,