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>

account/bp/actions/routes.py

@@ -0,0 +1,64 @@
+"""Account app action endpoints.
+
+Exposes write operations at ``/internal/actions/<action_name>`` for
+cross-app callers (blog webhooks) via the internal action client.
+"""
+from __future__ import annotations
+
+from quart import Blueprint, g, jsonify, request
+
+from shared.infrastructure.actions import ACTION_HEADER
+
+
+def register() -> Blueprint:
+    bp = Blueprint("actions", __name__, url_prefix="/internal/actions")
+
+    @bp.before_request
+    async def _require_action_header():
+        if not request.headers.get(ACTION_HEADER):
+            return jsonify({"error": "forbidden"}), 403
+
+    _handlers: dict[str, object] = {}
+
+    @bp.post("/<action_name>")
+    async def handle_action(action_name: str):
+        handler = _handlers.get(action_name)
+        if handler is None:
+            return jsonify({"error": "unknown action"}), 404
+        try:
+            result = await handler()
+            return jsonify(result)
+        except Exception as exc:
+            import logging
+            logging.getLogger(__name__).exception("Action %s failed", action_name)
+            return jsonify({"error": str(exc)}), 500
+
+    # --- ghost-sync-member ---
+    async def _ghost_sync_member():
+        """Sync a single Ghost member into db_account."""
+        data = await request.get_json()
+        ghost_id = data.get("ghost_id")
+        if not ghost_id:
+            return {"error": "ghost_id required"}, 400
+
+        from services.ghost_membership import sync_single_member
+        await sync_single_member(g.s, ghost_id)
+        return {"ok": True}
+
+    _handlers["ghost-sync-member"] = _ghost_sync_member
+
+    # --- ghost-push-member ---
+    async def _ghost_push_member():
+        """Push a local user's membership data to Ghost."""
+        data = await request.get_json()
+        user_id = data.get("user_id")
+        if not user_id:
+            return {"error": "user_id required"}, 400
+
+        from services.ghost_membership import sync_member_to_ghost
+        result_id = await sync_member_to_ghost(g.s, int(user_id))
+        return {"ok": True, "ghost_id": result_id}
+
+    _handlers["ghost-push-member"] = _ghost_push_member
+
+    return bp