feat(core-api): implement abstract data transformer architecture

- **Core API**: Introduced `IDataTransformer` interface and `BaseDataTransformer` abstract class.
- **Data Services**: Updated `BaseRemoteDataServices` and `DataServicesConfig` to support optional transformer injection. CRUD methods now auto-transform data mapping (DTO <-> Entity) when a transformer is provided.
- **Generics**: Updated `CommonRemoteDataServices` to forward the `TDTO` generic for strict type safety. Fixed `ApiResponse<T>` casting and excess property type checks.
- **Showcase/Samples**: Created `BookingTransformer` (snake_case to camelCase mapping) and injected it into `booking.data-services.ts`.
- **Advanced Showcase**: Added `AdvancedBookingTransformer` and `AdvancedBookingService` to demonstrate extending base transformers with custom methods (e.g., `getAvailabilityChart`).
- **Documentation**: Added comprehensive developer guide at `apps/docs-dev/src/packages/core-api/transformers.md`.
- **Testing**: Added 16 new unit tests for transformer integrations (identity and mock scenarios).

Note: The implementation is 100% backward compatible. All 49/49 tests pass and the TypeScript typecheck is entirely clean.

packages/core-api/src/data-services/common-remote.data-services.ts

@@ -12,11 +12,12 @@ import { BaseRemoteDataServices } from './base-remote.data-services';
  * CRUD + lifecycle, extend BaseRemoteDataServices instead and add
  * custom methods using `this.execute()` or `this.customRequest()`.
  *
- * @typeParam E - The domain entity type
+ * @typeParam E    - The domain entity type
+ * @typeParam TDTO - The API DTO shape (defaults to E for backward compatibility)
  *
  * @example
  * ```ts
- * // Direct instantiation for standard modules
+ * // Direct instantiation for standard modules (no transformer)
  * const bookingServices = new CommonRemoteDataServices<BookingEntity>(
  *   apiClient,
  *   { apiUrl: '/bookings', moduleKey: 'BOOKING' },
@@ -27,6 +28,19 @@ import { BaseRemoteDataServices } from './base-remote.data-services';
  *
  * @example
  * ```ts
+ * // With transformer for DTO ↔ Entity mapping
+ * const bookingServices = new CommonRemoteDataServices<BookingEntity, BookingDTO>(
+ *   apiClient,
+ *   {
+ *     apiUrl: '/bookings',
+ *     moduleKey: 'BOOKING',
+ *     transformer: new BookingTransformer(),
+ *   },
+ * );
+ * ```
+ *
+ * @example
+ * ```ts
  * // For modules needing custom operations, extend the base:
  * class InvoiceDataServices extends BaseRemoteDataServices<InvoiceEntity> {
  *   async calculateTax(invoiceId: string) {
@@ -40,4 +54,6 @@ import { BaseRemoteDataServices } from './base-remote.data-services';
  */
 export class CommonRemoteDataServices<
   E extends BaseEntity = BaseEntity,
-> extends BaseRemoteDataServices<E> {}
+  TDTO = E,
+> extends BaseRemoteDataServices<E, TDTO> {}
+