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>