feat(train-scheduling): implement day-level booking pool

- Added `unplaced` method in `BookingNotifierService` to log warnings for bookings that cannot be placed on any train.
- Introduced `getAvailableDays` method in `TrainSchedulingService` to retrieve distinct days with open departures for a given route.
- Created `AvailableDaysQueryDto` for querying available days based on origin and destination yards.
- Updated `TrainSchedulingController` to expose an endpoint for available days.
- Modified frontend components to support day-level booking, allowing customers to select only a day without pinning to a specific train.
- Removed references to train schedules in booking forms and review steps, emphasizing day selection.
- Added a database migration to create an index for efficient querying of bookings by route and day.

apps/edr-freight-api/src/modules/bookings/dto/create-booking.dto.ts

@@ -91,12 +91,21 @@ export class CreateBookingDto {
   @IsUUID()
   trainId?: string;
 
-  /** Target schedule this booking is created against (required by the backoffice create form). */
-  @ApiPropertyOptional({ format: 'uuid', description: 'Target train schedule (pool membership)' })
+  /**
+   * Staff-only manual pin to a specific train. Customers omit this — they pick a
+   * DAY via {@link scheduledDate} and the batch engine assigns a train within
+   * that (route, day) pool. When provided, the schedule must be OPEN and on the
+   * booking route.
+   */
+  @ApiPropertyOptional({
+    format: 'uuid',
+    description: 'Staff only: pin to a specific train schedule. Customers omit this.',
+  })
   @IsOptional()
   @IsUUID()
   trainScheduleId?: string;
 
+  /** The day the customer wants to ship (the pool day key). */
   @ApiProperty({ example: '2026-06-15T00:00:00.000Z' })
   @IsDateString()
   scheduledDate!: string;