CRITICAL: Add missing function exports to user-manager.sh

Problem:
- user-manager.sh defined functions but NEVER exported them
- Functions worked when called directly but returned empty in nested calls
- calculate_server_memory_capacity showed 0 pools because get_user_domains returned empty
- Memory capacity output showed garbled: 'pickledperilMB' instead of numbers

Root cause:
- When php-analyzer.sh called get_user_domains() inside a function,
  bash couldn't find the function because it wasn't exported
- Only exported functions are available in subshells/nested calls

Fix:
- Added export -f for ALL 14 user-manager functions
- Now functions work correctly when called from other libraries

Functions exported:
- list_all_users, list_cpanel_users, list_plesk_users, list_interworx_users, list_system_users
- get_user_info, get_user_domains, get_cpanel_user_domains, get_plesk_user_domains, get_interworx_user_domains
- get_user_databases, get_user_log_files, select_user_interactive, display_user_overview

Impact:
- Memory capacity analysis now works
- All domain iteration functions work correctly

lib/user-manager.sh

@@ -720,3 +720,19 @@ show_all_users_summary() {
 
     echo ""
 }
+
+# Export all functions for use in other scripts
+export -f list_all_users
+export -f list_cpanel_users
+export -f list_plesk_users
+export -f list_interworx_users
+export -f list_system_users
+export -f get_user_info
+export -f get_user_domains
+export -f get_cpanel_user_domains
+export -f get_plesk_user_domains
+export -f get_interworx_user_domains
+export -f get_user_databases
+export -f get_user_log_files
+export -f select_user_interactive
+export -f display_user_overview