feat: MercadoLibre mejoras - importar existentes, sync stock, sync ordenes

- meli_service.py: agrega get_user_items() para obtener publicaciones del vendedor
- marketplace_external_service.py:
  - import_existing_listings(): importa publicaciones existentes de ML a marketplace_listings
  - process_meli_sync_queue(): procesa cola de sincronizacion de stock a ML
  - Actualiza stock en ML via update_item(available_quantity)
- marketplace_external_bp.py:
  - POST /listings/import-existing - importa publicaciones existentes
  - POST /sync-stock - procesa cola de stock manualmente
  - POST /orders/sync - sincroniza ordenes manualmente
- inventory_engine.py: inserta en meli_sync_queue tras cada operacion de inventario
- migration v4.2: crea tabla meli_sync_queue

Prueba en tenant_refaccionaria_rached: 52 publicaciones importadas exitosamente

pos/blueprints/marketplace_external_bp.py

@@ -235,6 +235,26 @@ def create_listings():
         conn.close()
 
 
+@marketplace_ext_bp.route("/listings/import-existing", methods=["POST"])
+@require_auth()
+def import_existing_listings():
+    """Import all existing MercadoLibre listings for the connected seller."""
+    err = _require_meli_manage()
+    if err:
+        return err
+
+    conn = get_tenant_conn(g.tenant_id)
+    try:
+        result = meli_svc.import_existing_listings(conn)
+        return jsonify(result), 200
+    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("/inventory-check", methods=["POST"])
 @require_auth()
 def inventory_check():
@@ -254,6 +274,26 @@ def inventory_check():
         conn.close()
 
 
+@marketplace_ext_bp.route("/sync-stock", methods=["POST"])
+@require_auth()
+def sync_stock_to_meli():
+    """Process pending stock updates to MercadoLibre."""
+    err = _require_meli_manage()
+    if err:
+        return err
+
+    conn = get_tenant_conn(g.tenant_id)
+    try:
+        result = meli_svc.process_meli_sync_queue(conn)
+        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("/categories/<category_id>/attributes", methods=["GET"])
 @require_auth()
 def category_attributes(category_id):
@@ -526,6 +566,26 @@ def answer_question(question_id):
 # ORDERS
 # ═══════════════════════════════════════════════════════════════════════════
 
+@marketplace_ext_bp.route("/orders/sync", methods=["POST"])
+@require_auth()
+def sync_orders():
+    """Manually trigger sync of MercadoLibre orders."""
+    err = _require_meli_manage()
+    if err:
+        return err
+
+    conn = get_tenant_conn(g.tenant_id)
+    try:
+        result = meli_svc.fetch_and_save_orders(conn)
+        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("/orders", methods=["GET"])
 @require_auth()
 def list_orders():