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/blueprints/marketplace_external_bp.py

@@ -183,6 +183,7 @@ def create_listings():
     category_id = data.get("category_id")
     listing_type = data.get("listing_type", "gold_special")
     shipping_mode = data.get("shipping_mode", "me2")
+    custom_data = data.get("custom_data", {})
 
     if not inventory_ids:
         return jsonify({"error": "inventory_ids required"}), 400
@@ -197,6 +198,7 @@ def create_listings():
             meli_category_id=category_id,
             listing_type_id=listing_type,
             shipping_mode=shipping_mode,
+            custom_data=custom_data,
         )
         return jsonify(result), 201
     except ValueError as e:
@@ -207,6 +209,134 @@ def create_listings():
         conn.close()
 
 
+@marketplace_ext_bp.route("/inventory-check", methods=["POST"])
+@require_auth()
+def inventory_check():
+    """Check local pre-flight status for ML publishing (duplicates, stock, price, image)."""
+    data = request.get_json() or {}
+    inventory_ids = data.get("inventory_ids", [])
+    if not inventory_ids:
+        return jsonify({"error": "inventory_ids required"}), 400
+
+    conn = get_tenant_conn(g.tenant_id)
+    try:
+        result = meli_svc.check_inventory_ml_status(conn, inventory_ids)
+        return jsonify(result)
+    except Exception as e:
+        return jsonify({"error": str(e)}), 500
+    finally:
+        conn.close()
+
+
+@marketplace_ext_bp.route("/categories/<category_id>/attributes", methods=["GET"])
+@require_auth()
+def category_attributes(category_id):
+    """Get required attributes for a MercadoLibre category."""
+    conn = get_tenant_conn(g.tenant_id)
+    try:
+        cfg = meli_svc.get_meli_config(conn)
+        svc = meli_svc._get_meli_service(cfg)
+        if not svc:
+            return jsonify({"error": "MercadoLibre not connected"}), 400
+        attrs = svc.get_category_attributes(category_id)
+        # Filter to required attributes only for the UI
+        required = [a for a in attrs if a.get("tags", {}).get("required")]
+        return jsonify({"attributes": required, "all": attrs})
+    except Exception as e:
+        return jsonify({"error": str(e)}), 500
+    finally:
+        conn.close()
+
+
+@marketplace_ext_bp.route("/listings/validate", methods=["POST"])
+@require_auth()
+def validate_listings():
+    """Validate items payload against ML /items/validate without creating them."""
+    err = _require_meli_manage()
+    if err:
+        return err
+
+    data = request.get_json() or {}
+    inventory_ids = data.get("inventory_ids", [])
+    category_id = data.get("category_id")
+    listing_type = data.get("listing_type", "gold_special")
+    shipping_mode = data.get("shipping_mode", "me2")
+    custom_data = data.get("custom_data", {})
+
+    if not inventory_ids:
+        return jsonify({"error": "inventory_ids required"}), 400
+    if not category_id:
+        return jsonify({"error": "category_id required"}), 400
+
+    conn = get_tenant_conn(g.tenant_id)
+    try:
+        result = meli_svc.validate_items(
+            conn,
+            inventory_ids=inventory_ids,
+            meli_category_id=category_id,
+            listing_type_id=listing_type,
+            shipping_mode=shipping_mode,
+            custom_data=custom_data,
+        )
+        return jsonify(result)
+    except ValueError as e:
+        return jsonify({"error": str(e)}), 400
+    except Exception as e:
+        return jsonify({"error": str(e)}), 500
+    finally:
+        conn.close()
+
+
+@marketplace_ext_bp.route("/listings/async", methods=["POST"])
+@require_auth()
+def create_listings_async():
+    """Enqueue ML publishing as a Celery background task."""
+    err = _require_meli_manage()
+    if err:
+        return err
+
+    data = request.get_json() or {}
+    inventory_ids = data.get("inventory_ids", [])
+    category_id = data.get("category_id")
+    listing_type = data.get("listing_type", "gold_special")
+    shipping_mode = data.get("shipping_mode", "me2")
+    custom_data = data.get("custom_data", {})
+
+    if not inventory_ids:
+        return jsonify({"error": "inventory_ids required"}), 400
+    if not category_id:
+        return jsonify({"error": "category_id required"}), 400
+
+    try:
+        from tasks import publish_meli_items_task
+        task = publish_meli_items_task.delay(
+            g.tenant_id,
+            inventory_ids=inventory_ids,
+            category_id=category_id,
+            listing_type=listing_type,
+            shipping_mode=shipping_mode,
+            custom_data=custom_data,
+        )
+        return jsonify({"task_id": task.id, "status": "queued"}), 202
+    except Exception as e:
+        return jsonify({"error": str(e)}), 500
+
+
+@marketplace_ext_bp.route("/listings/async/<task_id>", methods=["GET"])
+@require_auth()
+def get_async_listing_status(task_id):
+    """Get status of an async ML publishing task."""
+    try:
+        from celery.result import AsyncResult
+        from app import celery as celery_app
+        result = AsyncResult(task_id, app=celery_app)
+        if result.ready():
+            return jsonify({"status": "done", "result": result.result or {}})
+        return jsonify({"status": "pending"})
+    except Exception as e:
+        return jsonify({"error": str(e)}), 500
+
+
 @marketplace_ext_bp.route("/listings/<int:listing_id>/sync", methods=["POST"])
 @require_auth()
 def sync_listing(listing_id):