Fix Product filtering and add kit disassembly functionality

Fixed:
- Replace is_active with status='active' for Product filtering in IncomingModelForm
- Product model uses status field instead of is_active

Added:
- Showcase field to ProductKit for tracking showcase placement
- product_kit field to Reservation for tracking kit-specific reservations
- Disassemble button in POS terminal for showcase kits
- API endpoint for kit disassembly (release reservations, mark discontinued)
- Improved reservation filtering when dismantling specific kits

Changes:
- ShowcaseManager now links reservations to specific kit instances
- POS terminal modal shows disassemble button in edit mode
- Kit disassembly properly updates stock aggregates

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

myproject/pos/templates/pos/terminal.html

@@ -252,6 +252,12 @@
         </div>
       </div>
       <div class="modal-footer">
+        <!-- Кнопка "Разобрать" (отображается только в режиме редактирования) -->
+        <button type="button" class="btn btn-danger me-auto" id="disassembleKitBtn" style="display: none;">
+          <i class="bi bi-scissors"></i> Разобрать букет
+        </button>
+
+        <!-- Правая группа кнопок -->
         <button type="button" class="btn btn-secondary" data-bs-dismiss="modal">Отмена</button>
         <button type="button" class="btn btn-primary" id="confirmCreateTempKit">
           <i class="bi bi-check-circle"></i> Создать и зарезервировать