Add assignment dropdown on ticket creation and fix Discord webhook URLs

- Add APP_DOMAIN config for correct Discord webhook ticket links
- Add "Assign To" dropdown on create ticket form
- Update TicketModel.createTicket() to support assigned_to field
- Update documentation for APP_DOMAIN requirement

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

models/TicketModel.php

@@ -376,8 +376,8 @@ class TicketModel {
             error_log("Ticket ID generation used fallback after {$maxAttempts} random attempts");
         }
 
-        $sql = "INSERT INTO tickets (ticket_id, title, description, status, priority, category, type, created_by, visibility, visibility_groups)
-                VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?)";
+        $sql = "INSERT INTO tickets (ticket_id, title, description, status, priority, category, type, created_by, assigned_to, visibility, visibility_groups)
+                VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)";
 
         $stmt = $this->conn->prepare($sql);
 
@@ -388,6 +388,7 @@ class TicketModel {
         $type = $ticketData['type'] ?? 'Issue';
         $visibility = $ticketData['visibility'] ?? 'public';
         $visibilityGroups = $ticketData['visibility_groups'] ?? null;
+        $assignedTo = !empty($ticketData['assigned_to']) ? (int)$ticketData['assigned_to'] : null;
 
         // Validate visibility
         $allowedVisibilities = ['public', 'internal', 'confidential'];
@@ -409,7 +410,7 @@ class TicketModel {
         }
 
         $stmt->bind_param(
-            "sssssssiss",
+            "sssssssiiss",
             $ticket_id,
             $ticketData['title'],
             $ticketData['description'],
@@ -418,6 +419,7 @@ class TicketModel {
             $category,
             $type,
             $createdBy,
+            $assignedTo,
             $visibility,
             $visibilityGroups
         );