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/registry.py

@@ -23,6 +23,7 @@ from shared.contracts.protocols import (
     CartService,
     FederationService,
 )
+from shared.contracts.likes import LikesService
 
 
 class _ServiceRegistry:
@@ -38,6 +39,7 @@ class _ServiceRegistry:
         self._market: MarketService | None = None
         self._cart: CartService | None = None
         self._federation: FederationService | None = None
+        self._likes: LikesService | None = None
         self._extra: dict[str, Any] = {}
 
     # -- calendar -------------------------------------------------------------
@@ -73,6 +75,17 @@ class _ServiceRegistry:
     def cart(self, impl: CartService) -> None:
         self._cart = impl
 
+    # -- likes ----------------------------------------------------------------
+    @property
+    def likes(self) -> LikesService:
+        if self._likes is None:
+            raise RuntimeError("LikesService not registered")
+        return self._likes
+
+    @likes.setter
+    def likes(self, impl: LikesService) -> None:
+        self._likes = impl
+
     # -- federation -----------------------------------------------------------
     @property
     def federation(self) -> FederationService: