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>

relations/bp/fragments/routes.py

@@ -47,6 +47,7 @@ def register():
         container_type = request.args.get("container_type", "page")
         container_id = int(request.args.get("container_id", 0))
         post_slug = request.args.get("post_slug", "")
+        nav_class = request.args.get("nav_class", "")
         exclude_raw = request.args.get("exclude", "")
         exclude = set(exclude_raw.split(",")) if exclude_raw else set()
 
@@ -76,7 +77,7 @@ def register():
                     name=child.label or "",
                     icon=defn.nav_icon or "",
                     **{
-                        "nav-class": "",
+                        "nav-class": nav_class,
                         "relation-type": defn.name,
                     },
                 ))