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/post/routes.py

@@ -13,7 +13,7 @@ from .services.post_data import post_data
 from shared.infrastructure.data_client import fetch_data
 from shared.infrastructure.actions import call_action
 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
 
 from shared.browser.app.redis_cacher import cache_page, clear_cache
 
@@ -69,22 +69,12 @@ def register():
           db_post_id = (g.post_data.get("post") or {}).get("id")
           post_slug = (g.post_data.get("post") or {}).get("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,