harden production headers and probe contract

nginx.conf

@@ -9,6 +9,9 @@ server {
     add_header X-Content-Type-Options "nosniff" always;
     add_header Referrer-Policy "no-referrer" always;
     add_header Permissions-Policy "camera=(), microphone=(), geolocation=()" always;
+    add_header X-Frame-Options "DENY" always;
+    add_header Content-Security-Policy "default-src 'self'; base-uri 'self'; object-src 'none'; frame-ancestors 'none'; script-src 'self' https://accounts.google.com; style-src 'self' 'unsafe-inline' https://fonts.googleapis.com; font-src 'self' https://fonts.gstatic.com data:; img-src 'self' data: blob: https:; connect-src 'self' https:; worker-src 'self' blob:; frame-src 'self' https://accounts.google.com" always;
+    add_header Strict-Transport-Security "max-age=31536000; includeSubDomains" always;
 
     include /etc/nginx/mime.types;
     # Vite, pdf.js worker'ını .mjs uzantısıyla üretiyor; Debian'ın mime.types
@@ -24,6 +27,11 @@ server {
         proxy_http_version 1.1;
         proxy_set_header Host $host;
         proxy_set_header X-Real-IP $remote_addr;
+        # Node and nginx share the same security policy. Keep one canonical
+        # header value at the edge instead of returning comma-duplicated values.
+        proxy_hide_header X-Content-Type-Options;
+        proxy_hide_header Referrer-Policy;
+        proxy_hide_header Permissions-Policy;
         # Belge yükleme: arayüz 10 MB'a izin veriyor, base64 ile ~13,4 MB gövde.
         # nginx varsayılanı 1 MB → 1 MB üstü her dosya 413 ile "yüklenemedi" oluyordu.
         client_max_body_size 20m;