Decouple all cross-app service calls to HTTP endpoints

Replace every direct cross-app services.* call with HTTP-based
communication: call_action() for writes, fetch_data() for reads.
Each app now registers only its own domain service.

Infrastructure:
- shared/infrastructure/actions.py — POST client for /internal/actions/
- shared/infrastructure/data_client.py — GET client for /internal/data/
- shared/contracts/dtos.py — dto_to_dict/dto_from_dict serialization

Action endpoints (writes):
- events: 8 handlers (ticket adjust, claim/confirm, toggle, adopt)
- market: 2 handlers (create/soft-delete marketplace)
- cart: 1 handler (adopt cart for user)

Data endpoints (reads):
- blog: 4 (post-by-slug/id, posts-by-ids, search-posts)
- events: 10 (pending entries/tickets, entries/tickets for page/order,
  entry-ids, associated-entries, calendars, visible-entries-for-period)
- market: 1 (marketplaces-for-container)
- cart: 1 (cart-summary)

Service registration cleanup:
- blog→blog+federation, events→calendar+federation,
  market→market+federation, cart→cart only,
  federation→federation only, account→nothing
- Stubs reduced to minimal StubFederationService

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

federation/app.py

@@ -21,17 +21,23 @@ async def federation_context() -> dict:
     from shared.services.navigation import get_navigation_tree
     from shared.infrastructure.cart_identity import current_cart_identity
     from shared.infrastructure.fragments import fetch_fragments
+    from shared.infrastructure.data_client import fetch_data
+    from shared.contracts.dtos import CartSummaryDTO, dto_from_dict
 
     ctx = await base_context()
 
     # Fallback for _nav.html when nav-tree fragment fetch fails
     ctx["menu_items"] = await get_navigation_tree(g.s)
 
-    # Cart data (consistent with all other apps)
+    # Cart data via internal data endpoint
     ident = current_cart_identity()
-    summary = await services.cart.cart_summary(
-        g.s, user_id=ident["user_id"], session_id=ident["session_id"],
-    )
+    summary_params = {}
+    if ident["user_id"] is not None:
+        summary_params["user_id"] = ident["user_id"]
+    if ident["session_id"] is not None:
+        summary_params["session_id"] = ident["session_id"]
+    raw = await fetch_data("cart", "cart-summary", params=summary_params, required=False)
+    summary = dto_from_dict(CartSummaryDTO, raw) if raw else CartSummaryDTO()
     ctx["cart_count"] = summary.count + summary.calendar_count + summary.ticket_count
     ctx["cart_total"] = float(summary.total + summary.calendar_total + summary.ticket_total)