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/services/__init__.py

@@ -7,21 +7,9 @@ def register_domain_services() -> None:
 
     Federation owns: ActorProfile, APActivity, APFollower, APInboxItem,
     APAnchor, IPFSPin.
-    Standard deployment registers all services as real DB impls (shared DB).
+    Cross-app calls go over HTTP via call_action() / fetch_data().
     """
     from shared.services.registry import services
     from shared.services.federation_impl import SqlFederationService
-    from shared.services.blog_impl import SqlBlogService
-    from shared.services.calendar_impl import SqlCalendarService
-    from shared.services.market_impl import SqlMarketService
-    from shared.services.cart_impl import SqlCartService
 
     services.federation = SqlFederationService()
-    if not services.has("blog"):
-        services.blog = SqlBlogService()
-    if not services.has("calendar"):
-        services.calendar = SqlCalendarService()
-    if not services.has("market"):
-        services.market = SqlMarketService()
-    if not services.has("cart"):
-        services.cart = SqlCartService()