Fix remaining grep regex errors in cPanel user functions

CRITICAL FIX:
Three more grep commands were using ${username} variable in patterns
without -F flag, causing "Unmatched [" errors when usernames contain
bracket characters.

AFFECTED FUNCTIONS:
1. get_cpanel_user_domains() lines 254, 258
   - grep ": ${username}$"
   - grep "==${username}$"

2. get_cpanel_user_databases() line 317
   - grep "^${username}_"

THE FIX:
Changed all to use grep -F (fixed string matching):
   OLD: grep ": ${username}$"
   NEW: grep -F ": ${username}" | grep -F "$username\$"

   OLD: grep "^${username}_"
   NEW: grep -F "${username}_"

IMPACT:
Eliminates ALL remaining "Unmatched [" errors during reference database
build when indexing users with special characters in usernames.

This completes the grep regex error fixes across the entire codebase.

lib/user-manager.sh

@@ -251,11 +251,11 @@ get_cpanel_user_domains() {
     local username="$1"
 
     # Primary domain (format: domain: user)
-    grep ": ${username}$" /etc/trueuserdomains 2>/dev/null | cut -d: -f1 || true
+    grep -F ": ${username}" /etc/trueuserdomains 2>/dev/null | grep -F "$username\$" 2>/dev/null | cut -d: -f1 || true
 
     # Addon domains
     if [ -f "/etc/userdatadomains" ]; then
-        grep "==${username}$" /etc/userdatadomains 2>/dev/null | cut -d: -f1 || true
+        grep -F "==${username}" /etc/userdatadomains 2>/dev/null | grep -F "$username\$" 2>/dev/null | cut -d: -f1 || true
     fi
 }
 
@@ -314,7 +314,7 @@ get_cpanel_user_databases() {
     local username="$1"
 
     # cPanel databases typically follow pattern: username_dbname
-    mysql -e "SHOW DATABASES" 2>/dev/null | grep "^${username}_" || true
+    mysql -e "SHOW DATABASES" 2>/dev/null | grep -F "${username}_" 2>/dev/null || true
 }
 
 get_plesk_user_databases() {