Fix cross-DB queries: move page_configs to cart, fix OAuth code_hash lookup

page_configs table lives in db_cart but blog was querying it directly,
causing UndefinedTableError. Move all PageConfig read/write endpoints to
cart service and have blog proxy via fetch_data/call_action.

Also fix OAuth callback to use code_hash lookup (codes are now stored
hashed) and pass grant_token in redirect URL to prevent auth loops.

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

cart/bp/cart/global_routes.py

@@ -256,7 +256,7 @@ def register(url_prefix: str) -> Blueprint:
         if order.page_config_id:
             from shared.infrastructure.data_client import fetch_data
             from shared.contracts.dtos import CalendarEntryDTO, TicketDTO, dto_from_dict
-            raw_pc = await fetch_data("blog", "page-config-by-id",
+            raw_pc = await fetch_data("cart", "page-config-by-id",
                                       params={"id": order.page_config_id},
                                       required=False)
             post = await fetch_data("blog", "post-by-id",