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/tasks.py

@@ -267,3 +267,26 @@ def sync_vehicle_compatibility_task(self, tenant_id, item_id, part_number, name,
             tenant_conn.close()
         if master_conn:
             master_conn.close()
+
+
+@celery.task(bind=True)
+def publish_meli_items_task(self, tenant_id: int, inventory_ids: list, category_id: str, listing_type: str = "gold_special", shipping_mode: str = "me2", custom_data: dict = None):
+    """Publish inventory items to MercadoLibre asynchronously."""
+    from services import marketplace_external_service as meli_svc
+    from tenant_db import get_tenant_conn
+
+    conn = get_tenant_conn(tenant_id)
+    try:
+        result = meli_svc.publish_items(
+            conn,
+            inventory_ids=inventory_ids,
+            meli_category_id=category_id,
+            listing_type_id=listing_type,
+            shipping_mode=shipping_mode,
+            custom_data=custom_data or {},
+        )
+        return result
+    except Exception as e:
+        return {"success": [], "failed": [{"inventory_id": "batch", "error": str(e)}]}
+    finally:
+        conn.close()