chore: configure env-based DAV_ADMIN_PASSWORD and add security headers

Ultraworked with Sisyphus (https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

sabredav/server.php

@@ -144,11 +144,11 @@ if ($stmt->fetchColumn() == 0) {
     ");
 }
 
-// Create default user if not exists (username: admin, password: admin)
+// Create default user if not exists (username: admin, password from DAV_ADMIN_PASSWORD env)
+$davPassword = getenv('DAV_ADMIN_PASSWORD') ?: 'admin';
 $stmt = $pdo->query("SELECT COUNT(*) FROM dav_users");
 if ($stmt->fetchColumn() == 0) {
-    // MD5 hash of "admin:SabreDAV:admin"
-    $digest = md5('admin:SabreDAV:admin');
+    $digest = md5('admin:SabreDAV:' . $davPassword);
     $pdo->exec("
         INSERT INTO dav_users (username, digesta1) 
         VALUES ('admin', '$digest')