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_entry/routes.py

@@ -136,7 +136,7 @@ def register():
         nav_oobs = []
         for post in entry_posts:
             # Get associated entries for this post
-            from ..post.services.entry_associations import get_associated_entries
+            from shared.services.entry_associations import get_associated_entries
             associated_entries = await get_associated_entries(g.s, post.id)
 
             # Load calendars for this post