fix: member status visibility, team creation improvements, join approval flow

- Fix other members' status not visible due to users collection viewRule restriction
- Fix empty status treated as 'away' instead of 'idle' in membersByStatus
- Auto-set creator to 'in_team' status when creating team session
- Filter current user from idle members invite list
- Fix group store isGroupOwner using pb.authStore instead of localStorage
- Add nginx no-cache headers for index.html
- Add join_requests collection migration and join approval flow
- Update groups collection rules and add requireApproval field
- Add Memory types for Phase 2 planning

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/nginx.conf

@@ -10,6 +10,9 @@ server {
 
     location / {
         try_files $uri $uri/ /index.html;
+        add_header Cache-Control "no-cache, no-store, must-revalidate";
+        add_header Pragma "no-cache";
+        add_header Expires "0";
     }
 
     # API 代理到局域网 PocketBase