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/sx/primitives.py

@@ -516,6 +516,23 @@ def prim_parse_int(val: Any, default: Any = 0) -> int | Any:
         return default
 
 
+@register_primitive("parse-datetime")
+def prim_parse_datetime(val: Any) -> Any:
+    """``(parse-datetime "2024-01-15T10:00:00")`` → datetime object."""
+    from datetime import datetime
+    if not val or val is NIL:
+        return NIL
+    return datetime.fromisoformat(str(val))
+
+
+@register_primitive("split-ids")
+def prim_split_ids(val: Any) -> list[int]:
+    """``(split-ids "1,2,3")`` → [1, 2, 3]. Parse comma-separated int IDs."""
+    if not val or val is NIL:
+        return []
+    return [int(x.strip()) for x in str(val).split(",") if x.strip()]
+
+
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 # Assertions
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