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>

_config/split-databases.sh

@@ -42,6 +42,7 @@ DB_TABLES[db_blog]="
   menu_items
   menu_nodes
   container_relations
+  page_configs
 "
 
 DB_TABLES[db_market]="
@@ -78,7 +79,6 @@ DB_TABLES[db_events]="
   calendar_entry_posts
   ticket_types
   tickets
-  page_configs
 "
 
 DB_TABLES[db_federation]="