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/infrastructure/schema_blueprint.py

@@ -0,0 +1,31 @@
+"""Schema endpoint for inter-service protocol introspection.
+
+Every service exposes ``GET /internal/schema`` which returns a JSON
+manifest of all defquery and defaction definitions with their parameter
+signatures and docstrings.
+
+Usage::
+
+    from shared.infrastructure.schema_blueprint import create_schema_blueprint
+    app.register_blueprint(create_schema_blueprint("events"))
+"""
+
+from __future__ import annotations
+
+from quart import Blueprint, jsonify, request
+
+
+def create_schema_blueprint(service_name: str) -> Blueprint:
+    """Create a blueprint exposing ``/internal/schema``."""
+    bp = Blueprint(
+        f"schema_{service_name}",
+        __name__,
+        url_prefix="/internal",
+    )
+
+    @bp.get("/schema")
+    async def get_schema():
+        from shared.sx.query_registry import schema_for_service
+        return jsonify(schema_for_service(service_name))
+
+    return bp