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>

backend/pb_migrations/1776421645_created_groups.js

@@ -87,7 +87,7 @@ migrate((db) => {
     "listRule": "@request.auth.id != \"\"",
     "viewRule": "@request.auth.id != \"\"",
     "createRule": "@request.auth.id != \"\"",
-    "updateRule": "owner = @request.auth.id",
+    "updateRule": "@request.auth.id != \"\"",
     "deleteRule": "owner = @request.auth.id",
     "options": {}
   });