CRITICAL: Fix grep regex errors when usernames contain special characters

ROOT CAUSE:
Usernames containing bracket characters like '[' or ']' were being used
directly in grep patterns, causing:
  grep: Unmatched [, [^, [:, [., or [=

This happened during "Indexing users" when the reference database builder
called get_user_domains/get_user_databases with usernames containing brackets.

AFFECTED FUNCTIONS (lib/user-manager.sh):
- get_interworx_user_domains() line 284: grep -v "^${username}\."
- get_interworx_user_info() line 195: grep -A20 with $primary_domain
- get_user_processes() line 583: grep "^${username}"
- get_user_top_processes() line 590: grep "^${username}"

AFFECTED FUNCTIONS (lib/reference-db.sh):
- index_wordpress_sites() line 420: grep "^USER|${username}|"

THE FIX:
Changed all grep commands using variables in patterns to use -F (fixed string)
flag instead of regex matching, and added 2>/dev/null error suppression:

  OLD: grep "^${username}"
  NEW: grep -F "$username" 2>/dev/null

  OLD: grep -v "^${username}\."
  NEW: grep -vF "${username}." 2>/dev/null

IMPACT:
Eliminates ALL "Unmatched [" errors during reference database build,
even when usernames contain special regex characters: [].*+?^$(){}|

lib/reference-db.sh

@@ -417,7 +417,7 @@ build_wordpress_section() {
         # Check for common domain folder patterns
         if [[ "$path_after_home" == public_html ]]; then
             # This is the primary domain - get it from user info
-            domain=$(grep "^USER|${username}|" "$SYSREF_DB" | cut -d'|' -f3 || true)
+            domain=$(grep -F "USER|${username}|" "$SYSREF_DB" 2>/dev/null | cut -d'|' -f3 || true)
         elif [[ "$path_after_home" =~ ^public_html/(.+) ]]; then
             # Could be subdomain or subdirectory - extract folder name
             local folder=$(echo "$path_after_home" | cut -d'/' -f2)