Replace inter-service _handlers dicts with declarative sx defquery/defaction

The inter-service data layer (fetch_data/call_action) was the least
structured part of the codebase — Python _handlers dicts with ad-hoc
param extraction scattered across 16 route files. This replaces them
with declarative .sx query/action definitions that make the entire
inter-service protocol self-describing and greppable.

Infrastructure:
- defquery/defaction special forms in the sx evaluator
- Query/action registry with load, lookup, and schema introspection
- Query executor using async_eval with I/O primitives
- Blueprint factories (create_data_blueprint/create_action_blueprint)
  with sx-first dispatch and Python fallback
- /internal/schema endpoint on every service
- parse-datetime and split-ids primitives for type coercion

Service extractions:
- LikesService (toggle, is_liked, liked_slugs, liked_ids)
- PageConfigService (ensure, get_by_container, get_by_id, get_batch, update)
- RelationsService (wraps module-level functions)
- AccountDataService (user_by_email, newsletters)
- CartItemsService, MarketDataService (raw SQLAlchemy lookups)

50 of 54 handlers converted to sx, 4 Python fallbacks remain
(ghost-sync/push-member, clear-cart-for-order, create-order).
Net: -1,383 lines Python, +251 lines modified.

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

shared/services/account_impl.py

@@ -0,0 +1,42 @@
+"""Service methods for account data queries.
+
+Extracted from account/bp/data/routes.py to enable sx defquery conversion.
+"""
+from __future__ import annotations
+
+from typing import Any
+
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from shared.models import User
+
+
+class SqlAccountDataService:
+
+    async def user_by_email(
+        self, session: AsyncSession, *, email: str,
+    ) -> dict[str, Any] | None:
+        email = (email or "").strip().lower()
+        if not email:
+            return None
+        result = await session.execute(
+            select(User.id).where(User.email.ilike(email))
+        )
+        row = result.first()
+        if not row:
+            return None
+        return {"user_id": row[0]}
+
+    async def newsletters(self, session: AsyncSession) -> list[dict[str, Any]]:
+        from shared.models.ghost_membership_entities import GhostNewsletter
+        result = await session.execute(
+            select(
+                GhostNewsletter.id, GhostNewsletter.ghost_id,
+                GhostNewsletter.name, GhostNewsletter.slug,
+            ).order_by(GhostNewsletter.name)
+        )
+        return [
+            {"id": row[0], "ghost_id": row[1], "name": row[2], "slug": row[3]}
+            for row in result.all()
+        ]