fix: Enable proper sorting for Created By and Assigned To columns

Fixed server-side sorting for user-related columns on dashboard:

Problem:
- Clicking "Created By" or "Assigned To" headers didn't sort
- Columns were missing from $allowedColumns validation
- Fell back to ticket_id sort, appearing random to users

Solution:
1. Added 'created_by' and 'assigned_to' to $allowedColumns array

2. Smart sort expression mapping:
   - created_by → sorts by display_name/username (not user ID)
   - assigned_to → uses CASE to put unassigned at end, then sorts by name
   - Other columns → use table prefix (t.column_name)

3. Database-level NULL handling for assigned_to:
   - Uses CASE WHEN to sort unassigned tickets last
   - Regardless of ASC/DESC direction
   - Then alphabetically sorts assigned users

Result:
- A→Z: Alice, Bob, Charlie... Unassigned
- Z→A: Zack, Yolanda, Xavier... Unassigned
- Consistent grouping and predictable order

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

models/TicketModel.php

@@ -151,11 +151,23 @@ class TicketModel {
         }
         
         // Validate sort column to prevent SQL injection
-        $allowedColumns = ['ticket_id', 'title', 'status', 'priority', 'category', 'type', 'created_at', 'updated_at'];
+        $allowedColumns = ['ticket_id', 'title', 'status', 'priority', 'category', 'type', 'created_at', 'updated_at', 'created_by', 'assigned_to'];
         if (!in_array($sortColumn, $allowedColumns)) {
             $sortColumn = 'ticket_id';
         }
 
+        // Map column names to actual sort expressions
+        // For user columns, sort by display name with NULL handling for unassigned
+        $sortExpression = $sortColumn;
+        if ($sortColumn === 'created_by') {
+            $sortExpression = "COALESCE(u_created.display_name, u_created.username, 'System')";
+        } elseif ($sortColumn === 'assigned_to') {
+            // Put unassigned (NULL) at the end regardless of sort direction
+            $sortExpression = "CASE WHEN t.assigned_to IS NULL THEN 1 ELSE 0 END, COALESCE(u_assigned.display_name, u_assigned.username)";
+        } else {
+            $sortExpression = "t.$sortColumn";
+        }
+
         // Validate sort direction
         $sortDirection = strtolower($sortDirection) === 'asc' ? 'ASC' : 'DESC';
 
@@ -181,7 +193,7 @@ class TicketModel {
                 LEFT JOIN users u_created ON t.created_by = u_created.user_id
                 LEFT JOIN users u_assigned ON t.assigned_to = u_assigned.user_id
                 $whereClause
-                ORDER BY $sortColumn $sortDirection
+                ORDER BY $sortExpression $sortDirection
                 LIMIT ? OFFSET ?";
         $stmt = $this->conn->prepare($sql);