security: fix all LOW priority items from TODO.SECURITY.md

Item 13 — Remove deprecated X-XSS-Protection header
- nginx/posterg.conf: header removed (was '1; mode=block')
- nginx/SECURITY_HEADERS.md: new file documenting header decisions
  and explaining why X-XSS-Protection is counterproductive

Item 14 — Add rel="noreferrer" to external target="_blank" link
- public/admin/thanks.php: rel="noopener" → rel="noopener noreferrer"

Item 15 — Explicit (int) casts on all integer HTML outputs
- public/index.php: (int) on item id, page numbers
- public/search.php: (int) on totalItems, year options, item id, pagination

Item 16 — Remove unused DATABASE_PATH constant
- config/bootstrap.php: define('DATABASE_PATH', ...) removed

docs/TODO.SECURITY.md updated: items 13-16 marked resolved and
moved to the ✅ Resolved section.

config/bootstrap.php

@@ -6,9 +6,6 @@
 // Define application root
 define('APP_ROOT', dirname(__DIR__));
 
-// Database path
-define('DATABASE_PATH', APP_ROOT . '/database/test.db');
-
 // Error reporting
 if (php_sapi_name() === 'cli-server') {
     // Development mode