Move entry_associations to shared — fix events cross-app import

entry_associations only uses HTTP fetch_data/call_action, no direct DB.
Events app imported it via ..post.services which doesn't exist in events.

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

events/bp/calendar/routes.py

@@ -224,7 +224,7 @@ def register():
         html = await render_template("_types/calendars/index.html")
 
         if post_data:
-            from ..post.services.entry_associations import get_associated_entries
+            from shared.services.entry_associations import get_associated_entries
 
             post_id = (post_data.get("post") or {}).get("id")
             cals = (