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>

blog/bp/post/admin/routes.py

@@ -465,17 +465,18 @@ def register():
   @require_post_author
   async def edit(slug: str):
     from ...blog.ghost.ghost_posts import get_post_for_edit
-    from shared.models.ghost_membership_entities import GhostNewsletter
-    from sqlalchemy import select as sa_select
+    from shared.infrastructure.data_client import fetch_data
 
     ghost_id = g.post_data["post"]["ghost_id"]
     is_page = bool(g.post_data["post"].get("is_page"))
     ghost_post = await get_post_for_edit(ghost_id, is_page=is_page)
     save_success = request.args.get("saved") == "1"
 
-    newsletters = (await g.s.execute(
-        sa_select(GhostNewsletter).order_by(GhostNewsletter.name)
-    )).scalars().all()
+    # Newsletters live in db_account — fetch via HTTP
+    raw_newsletters = await fetch_data("account", "newsletters", required=False) or []
+    # Convert dicts to objects with .name/.ghost_id attributes for template compat
+    from types import SimpleNamespace
+    newsletters = [SimpleNamespace(**nl) for nl in raw_newsletters]
 
     if not is_htmx_request():
       html = await render_template(