feat: duplicate link action, watcher migration, fulltext search migration

- CreateTicketView: "Link as duplicate" button on each duplicate result;
  stores chosen ticket ID in hidden field, auto-creates duplicates dependency
  after ticket is saved (TicketController)
- migrations/004: ticket_watchers table (ticket_id, user_id primary key)
- migrations/005: FULLTEXT index on tickets(title, description) for fast
  relevance search replacing LIKE scan

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

controllers/TicketController.php

@@ -125,6 +125,17 @@ class TicketController {
                     $GLOBALS['auditLog']->logTicketCreate($userId, $result['ticket_id'], $ticketData);
                 }
 
+                // Auto-link as duplicate if requested from create form
+                $linkDupOf = isset($_POST['link_duplicate_of']) ? (int)$_POST['link_duplicate_of'] : 0;
+                if ($linkDupOf > 0) {
+                    $depSql = "INSERT IGNORE INTO ticket_dependencies (ticket_id, depends_on_ticket_id, dependency_type, created_by)
+                               VALUES (?, ?, 'duplicates', ?)";
+                    $depStmt = $this->conn->prepare($depSql);
+                    $depStmt->bind_param("iii", $result['ticket_id'], $linkDupOf, $userId);
+                    $depStmt->execute();
+                    $depStmt->close();
+                }
+
                 // Send Matrix notification for new ticket
                 NotificationHelper::sendTicketNotification($result['ticket_id'], $ticketData, 'manual');