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>

views/TicketView.php

@@ -167,6 +167,18 @@ function formatDetails($details, $actionType) {
                         }
                         ?>
                     </div>
+                    <div class="ticket-assignment" style="margin-top: 0.5rem;">
+                        <label style="font-weight: 500; margin-right: 0.5rem;">Assigned to:</label>
+                        <select id="assignedToSelect" class="assignment-select" style="padding: 0.25rem 0.5rem; border-radius: 4px; border: 1px solid var(--border-color, #ddd);">
+                            <option value="">Unassigned</option>
+                            <?php foreach ($allUsers as $user): ?>
+                                <option value="<?php echo $user['user_id']; ?>"
+                                    <?php echo ($ticket['assigned_to'] == $user['user_id']) ? 'selected' : ''; ?>>
+                                    <?php echo htmlspecialchars($user['display_name'] ?? $user['username']); ?>
+                                </option>
+                            <?php endforeach; ?>
+                        </select>
+                    </div>
                 </div>
                 <div class="header-controls">
                     <div class="status-priority-group">