Add Ticket Assignment feature (Feature 2)

- Add assigned_to column support in TicketModel with assignTicket() and unassignTicket() methods
- Create assign_ticket.php API endpoint for assignment operations
- Update TicketController to load user list from UserModel
- Add assignment dropdown UI in TicketView
- Add JavaScript handler for assignment changes
- Integrate with audit log for assignment tracking

Users can now assign tickets to team members via dropdown selector.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

models/TicketModel.php

@@ -11,10 +11,13 @@ class TicketModel {
                 u_created.username as creator_username,
                 u_created.display_name as creator_display_name,
                 u_updated.username as updater_username,
-                u_updated.display_name as updater_display_name
+                u_updated.display_name as updater_display_name,
+                u_assigned.username as assigned_username,
+                u_assigned.display_name as assigned_display_name
                 FROM tickets t
                 LEFT JOIN users u_created ON t.created_by = u_created.user_id
                 LEFT JOIN users u_updated ON t.updated_by = u_updated.user_id
+                LEFT JOIN users u_assigned ON t.assigned_to = u_assigned.user_id
                 WHERE t.ticket_id = ?";
         $stmt = $this->conn->prepare($sql);
         $stmt->bind_param("i", $id);
@@ -283,4 +286,37 @@ class TicketModel {
             ];
         }
     }
+
+    /**
+     * Assign ticket to a user
+     *
+     * @param int $ticketId Ticket ID
+     * @param int $userId User ID to assign to
+     * @param int $assignedBy User ID performing the assignment
+     * @return bool Success status
+     */
+    public function assignTicket($ticketId, $userId, $assignedBy) {
+        $sql = "UPDATE tickets SET assigned_to = ?, updated_by = ?, updated_at = NOW() WHERE ticket_id = ?";
+        $stmt = $this->conn->prepare($sql);
+        $stmt->bind_param("iii", $userId, $assignedBy, $ticketId);
+        $result = $stmt->execute();
+        $stmt->close();
+        return $result;
+    }
+
+    /**
+     * Unassign ticket (set assigned_to to NULL)
+     *
+     * @param int $ticketId Ticket ID
+     * @param int $updatedBy User ID performing the unassignment
+     * @return bool Success status
+     */
+    public function unassignTicket($ticketId, $updatedBy) {
+        $sql = "UPDATE tickets SET assigned_to = NULL, updated_by = ?, updated_at = NOW() WHERE ticket_id = ?";
+        $stmt = $this->conn->prepare($sql);
+        $stmt->bind_param("ii", $updatedBy, $ticketId);
+        $result = $stmt->execute();
+        $stmt->close();
+        return $result;
+    }
 }