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>

events/bp/fragments/routes.py

@@ -9,6 +9,8 @@ from __future__ import annotations
 from quart import Blueprint, Response, g, render_template, request
 
 from shared.infrastructure.fragments import FRAGMENT_HEADER
+from shared.infrastructure.data_client import fetch_data
+from shared.contracts.dtos import PostDTO, dto_from_dict
 from shared.services.registry import services
 
 
@@ -139,7 +141,8 @@ def register():
             parts = []
             for s in slugs:
                 parts.append(f"<!-- fragment:{s} -->")
-                post = await services.blog.get_post_by_slug(g.s, s)
+                raw = await fetch_data("blog", "post-by-slug", params={"slug": s}, required=False)
+                post = dto_from_dict(PostDTO, raw) if raw else None
                 if post:
                     calendars = await services.calendar.calendars_for_container(
                         g.s, "page", post.id,
@@ -157,7 +160,8 @@ def register():
         # Single mode
         if not slug:
             return ""
-        post = await services.blog.get_post_by_slug(g.s, slug)
+        raw = await fetch_data("blog", "post-by-slug", params={"slug": slug}, required=False)
+        post = dto_from_dict(PostDTO, raw) if raw else None
         if not post:
             return ""
         calendars = await services.calendar.calendars_for_container(