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/contracts/likes.py

@@ -0,0 +1,31 @@
+"""Protocol for the Likes domain service."""
+from __future__ import annotations
+
+from typing import Protocol, runtime_checkable
+
+from sqlalchemy.ext.asyncio import AsyncSession
+
+
+@runtime_checkable
+class LikesService(Protocol):
+    async def is_liked(
+        self, session: AsyncSession, *,
+        user_id: int, target_type: str,
+        target_slug: str | None = None, target_id: int | None = None,
+    ) -> bool: ...
+
+    async def liked_slugs(
+        self, session: AsyncSession, *,
+        user_id: int, target_type: str,
+    ) -> list[str]: ...
+
+    async def liked_ids(
+        self, session: AsyncSession, *,
+        user_id: int, target_type: str,
+    ) -> list[int]: ...
+
+    async def toggle(
+        self, session: AsyncSession, *,
+        user_id: int, target_type: str,
+        target_slug: str | None = None, target_id: int | None = None,
+    ) -> bool: ...