fix: use item_id (singular) in market.removeFromAlbum call

VK API v5.199 requires item_id not item_ids for market.removeFromAlbum.
The wrong parameter name caused silent failures — products were not
actually removed from their old album when moved to a new one.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/tasks/vk_sync.py

@@ -221,7 +221,7 @@ def _sync_product(
             old_album_id = str(vk_p.album_id)
             _vk_post("market.removeFromAlbum", {
                 "owner_id": owner_id,
-                "item_ids": product.vk_product_id,
+                "item_id": product.vk_product_id,
                 "album_ids": old_album_id,
             }, token)
             resp_album = _vk_post("market.addToAlbum", {