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>

shared/services/registry.py

@@ -1,18 +1,17 @@
 """Typed singleton registry for domain services.
 
+Each app registers ONLY its own domain service.  Cross-app calls go
+over HTTP via ``call_action()`` (writes) and ``fetch_data()`` (reads).
+
 Usage::
 
     from shared.services.registry import services
 
-    # Register at app startup
+    # Register at app startup (own domain only)
     services.blog = SqlBlogService()
 
-    # Query anywhere
-    if services.has("calendar"):
-        entries = await services.calendar.pending_entries(session, ...)
-
-    # Or use stubs for absent domains
-    summary = await services.cart.cart_summary(session, ...)
+    # Use locally within the owning app
+    post = await services.blog.get_post_by_slug(session, slug)
 """
 from __future__ import annotations