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>

account/bp/auth/routes.py

@@ -123,7 +123,7 @@ def register(url_prefix="/auth"):
         sep = "&" if "?" in redirect_uri else "?"
         return redirect(
             f"{redirect_uri}{sep}code={code}&state={state}"
-            f"&account_did={account_did}"
+            f"&account_did={account_did}&grant_token={grant_token}"
         )
 
     # --- OAuth2 token exchange (for external clients like artdag) -------------