Add PHP 7.4+ type hints to helpers, models, and middleware

Added strict typing with parameter types, return types, and property
types across all core classes:
- helpers: Database, ErrorHandler, CacheHelper
- models: TicketModel, UserModel, WorkflowModel, TemplateModel, UserPreferencesModel
- middleware: RateLimitMiddleware, CsrfMiddleware, SecurityHeadersMiddleware

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

middleware/SecurityHeadersMiddleware.php

@@ -5,14 +5,14 @@
  * Applies security-related HTTP headers to all responses.
  */
 class SecurityHeadersMiddleware {
-    private static $nonce = null;
+    private static ?string $nonce = null;
 
     /**
      * Generate or retrieve the CSP nonce for this request
      *
      * @return string The nonce value
      */
-    public static function getNonce() {
+    public static function getNonce(): string {
         if (self::$nonce === null) {
             self::$nonce = base64_encode(random_bytes(16));
         }
@@ -22,7 +22,7 @@ class SecurityHeadersMiddleware {
     /**
      * Apply security headers to the response
      */
-    public static function apply() {
+    public static function apply(): void {
         $nonce = self::getNonce();
 
         // Content Security Policy - restricts where resources can be loaded from