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>

events/bp/calendar_entry/routes.py

@@ -215,16 +215,17 @@ def register():
                             ticket_type_id=tt.id,
                         )
 
-        # Fetch container nav from market (skip calendar — we're on a calendar page)
+        # Fetch container nav from relations (exclude calendar — we're on a calendar page)
         container_nav_html = ""
         post_data = getattr(g, "post_data", None)
         if post_data:
             post_id = post_data["post"]["id"]
             post_slug = post_data["post"]["slug"]
-            container_nav_html = await fetch_fragment("market", "container-nav", params={
+            container_nav_html = await fetch_fragment("relations", "container-nav", params={
                 "container_type": "page",
                 "container_id": str(post_id),
                 "post_slug": post_slug,
+                "exclude": "page->calendar",
             })
 
         return {