Decouple cross-domain DB queries for per-app database split

Move Ghost membership sync from blog to account service so blog no
longer queries account tables (users, ghost_labels, etc.). Account
runs membership sync at startup and exposes HTTP action/data endpoints
for webhook-triggered syncs and user lookups.

Key changes:
- account/services/ghost_membership.py: all membership sync functions
- account/bp/actions + data: ghost-sync-member, user-by-email, newsletters
- blog ghost_sync.py: stripped to content-only (posts, authors, tags)
- blog webhook member: delegates to account via call_action()
- try_publish: opens federation session when DBs differ
- oauth.py callback: uses get_account_session() for OAuthCode
- page_configs moved from db_events to db_blog in split script

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

shared/infrastructure/oauth.py

@@ -22,7 +22,7 @@ from quart import (
 )
 from sqlalchemy import select
 
-from shared.db.session import get_session
+from shared.db.session import get_session, get_account_session
 from shared.models.oauth_code import OAuthCode
 from shared.infrastructure.urls import account_url, app_url
 from shared.infrastructure.cart_identity import current_cart_identity
@@ -100,7 +100,8 @@ def create_oauth_blueprint(app_name: str) -> Blueprint:
         expected_redirect = app_url(app_name, "/auth/callback")
         now = datetime.now(timezone.utc)
 
-        async with get_session() as s:
+        # OAuthCode lives in db_account — use account session
+        async with get_account_session() as s:
             async with s.begin():
                 result = await s.execute(
                     select(OAuthCode)