Add plesk_list_users() function for Plesk user discovery

Issue: list_plesk_users() in user-manager.sh was trying to query MySQL
but the query was failing, resulting in 0 users detected on Plesk.

Fix:
1. Added plesk_list_users() to plesk-helpers.sh that uses:
   - Plesk CLI: 'plesk bin client --list' (primary)
   - Fallback: Scan /var/www/vhosts directories

2. Updated list_plesk_users() in user-manager.sh to:
   - First try plesk_list_users() if available
   - Then try MySQL query
   - Last resort: directory scan

This should now detect Plesk users from either Plesk API or
filesystem fallback.

Testing: Will verify on Plesk server

lib/plesk-helpers.sh

@@ -46,6 +46,24 @@ plesk_list_domains() {
     fi
 }
 
+#############################################################################
+# USER DISCOVERY
+#############################################################################
+
+# Get list of all Plesk users (clients)
+# Returns: One username per line
+plesk_list_users() {
+    if plesk_cli_available; then
+        # Try to get client logins from Plesk
+        plesk_exec bin client --list 2>/dev/null | tail -n +3 | awk '{print $1}' | grep -v "^$"
+    else
+        # Fallback: Get unique owners from vhosts directories
+        find /var/www/vhosts -maxdepth 1 -type d -printf "%f\n" 2>/dev/null | \
+            grep -v "^system$\|^chroot$\|^\.skel$\|^default$\|^fs$\|^fs-passwd$" | \
+            grep -v "^\." || true
+    fi
+}
+
 # Get domain info
 # Usage: plesk_domain_info DOMAIN
 plesk_domain_info() {