[security]KVE-2026-0701 .shtml 확장자 필터 누락으로 인한 RCE 수정

- bbs/write_update.php, bbs/qawrite_update.php: 업로드 블랙리스트에 shtml|shtm 추가
- install/install_db.php, data/.htaccess: FilesMatch 정규식에 [Ss]? 추가하여 .shtml 차단

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

data/.htaccess

@@ -0,0 +1,5 @@
+<FilesMatch "\.(htaccess|htpasswd|[Pp][Hh][Pp]|[Pp][Hh][Tt]|[Ss]?[Pp]?[Hh][Tt][Mm][Ll]?|[Ii][Nn][Cc]|[Cc][Gg][Ii]|[Pp][Ll]|[Pp][Hh][Aa][Rr]|[Ss][Vv][Gg][Zz]?)">
+Order allow,deny
+Deny from all
+</FilesMatch>
+RedirectMatch 403 /session/.*