Migrate callers from attach-child/detach-child to relate/unrelate API

Switch all cross-service relation calls to the new registry-aware
relate/unrelate/can-relate actions, and consolidate per-service
container-nav fragment fetches into the generic relations handler.

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

blog/bp/blog/routes.py

@@ -103,7 +103,7 @@ def register(url_prefix, title):
       from shared.infrastructure.cart_identity import current_cart_identity
       from shared.infrastructure.data_client import fetch_data
       from shared.contracts.dtos import CartSummaryDTO, dto_from_dict
-      from shared.infrastructure.fragments import fetch_fragment, fetch_fragments
+      from shared.infrastructure.fragments import fetch_fragment
 
       p_data = await _post_data("home", g.s, include_drafts=False)
       if not p_data:
@@ -116,22 +116,12 @@ def register(url_prefix, title):
       db_post_id = p_data["post"]["id"]
       post_slug = p_data["post"]["slug"]
 
-      # Fetch container nav fragments from events + market
-      paginate_url = url_for(
-          'blog.post.widget_paginate',
-          slug=post_slug, widget_domain='calendar',
-      )
-      nav_params = {
+      # Fetch container nav from relations service
+      container_nav_html = await fetch_fragment("relations", "container-nav", params={
           "container_type": "page",
           "container_id": str(db_post_id),
           "post_slug": post_slug,
-          "paginate_url": paginate_url,
-      }
-      events_nav_html, market_nav_html = await fetch_fragments([
-          ("events", "container-nav", nav_params),
-          ("market", "container-nav", nav_params),
-      ])
-      container_nav_html = events_nav_html + market_nav_html
+      })
 
       ctx = {
           **p_data,