feat: robust ML publish with pre-flight, preview, validation, async

- Add /inventory-check endpoint for local pre-flight validation
- Add /listings/validate endpoint using ML /items/validate API
- Add /categories/<id>/attributes endpoint for required attrs
- Add /listings/async + polling for background publishing via Celery
- Editable preview: title (0/60 counter), price, stock per item
- Pre-flight checks: image, stock, price, duplicate detection
- Image upload directly from publish modal (uses existing /items/<id>/image)
- Dynamic required attributes form based on selected ML category
- Frontend: validate button, async polling with progress, detailed error display
- Backend: build_item_payload supports custom_title, extra_attributes

pos/services/meli_service.py

@@ -145,6 +145,10 @@ class MeliService:
 
     # ─── Items (listings) ────────────────────────────────────────────────
 
+    def validate_item(self, payload: dict) -> dict:
+        """Validate an item payload without creating it."""
+        return self._request("POST", "/items/validate", json_payload=payload)
+
     def create_item(self, payload: dict) -> dict:
         return self._request("POST", "/items", json_payload=payload)