Add X-Content-Type-Options and Referrer-Policy security headers

Add two missing security headers to the existing securityHeadersMiddleware:

- X-Content-Type-Options: nosniff — prevents browsers from MIME-sniffing
  responses away from the declared Content-Type, stopping e.g. a
  user-uploaded image from being reinterpreted as executable HTML/JS

- Referrer-Policy: strict-origin-when-cross-origin — limits URL information
  leaked in the Referer header on cross-origin requests to just the origin
  (no path), and sends nothing at all on HTTPS-to-HTTP downgrades

backend/app/rest/api/rest.go

@@ -618,7 +618,14 @@ func cacheControl(expiration time.Duration, version string) func(http.Handler) h
 	}
 }
 
-// securityHeadersMiddleware sets security-related headers: Content-Security-Policy and Permissions-Policy
+// securityHeadersMiddleware sets security-related headers:
+// - Content-Security-Policy: controls which resources the browser is allowed to load
+// - Permissions-Policy: disables browser features (camera, mic, etc.) not needed by a comment widget
+// - X-Content-Type-Options: prevents browsers from MIME-sniffing responses away from the declared type,
+//   stopping e.g. a user-uploaded image from being reinterpreted as executable HTML/JS
+// - Referrer-Policy: controls how much URL information leaks in the Referer header on cross-origin
+//   requests; "strict-origin-when-cross-origin" sends only the origin (no path) to other domains
+//   and nothing at all on HTTPS→HTTP downgrades
 func securityHeadersMiddleware(imageProxyEnabled bool, allowedAncestors []string) func(http.Handler) http.Handler {
 	return func(next http.Handler) http.Handler {
 		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
@@ -632,6 +639,8 @@ func securityHeadersMiddleware(imageProxyEnabled bool, allowedAncestors []string
 			}
 			w.Header().Set("Content-Security-Policy", fmt.Sprintf("default-src 'none'; base-uri 'none'; form-action 'none'; connect-src 'self'; frame-src 'self' mailto:; img-src %s; script-src 'self' 'unsafe-inline'; style-src 'self' 'unsafe-inline'; font-src data:; object-src 'none'; frame-ancestors %s;", imgSrc, frameAncestors))
 			w.Header().Set("Permissions-Policy", "accelerometer=(), autoplay=(), camera=(), cross-origin-isolated=(), display-capture=(), encrypted-media=(), fullscreen=(), geolocation=(), gyroscope=(), keyboard-map=(), magnetometer=(), microphone=(), midi=(), payment=(), picture-in-picture=(), publickey-credentials-get=(), screen-wake-lock=(), sync-xhr=(), usb=(), xr-spatial-tracking=(), clipboard-read=(), clipboard-write=(), gamepad=(), hid=(), idle-detection=(), interest-cohort=(), serial=(), unload=(), window-management=()")
+			w.Header().Set("X-Content-Type-Options", "nosniff")
+			w.Header().Set("Referrer-Policy", "strict-origin-when-cross-origin")
 			next.ServeHTTP(w, r)
 		})
 	}

backend/app/rest/api/rest_test.go

@@ -352,6 +352,8 @@ func TestRest_securityHeaders(t *testing.T) {
 	defer resp.Body.Close()
 	assert.Equal(t, http.StatusOK, resp.StatusCode)
 	assert.Contains(t, resp.Header.Get("Content-Security-Policy"), "img-src *;")
+	assert.Equal(t, "nosniff", resp.Header.Get("X-Content-Type-Options"))
+	assert.Equal(t, "strict-origin-when-cross-origin", resp.Header.Get("Referrer-Policy"))
 	teardown()
 
 	// check CSP with proxy enabled
@@ -364,6 +366,8 @@ func TestRest_securityHeaders(t *testing.T) {
 	defer resp.Body.Close()
 	assert.Equal(t, http.StatusOK, resp.StatusCode)
 	assert.Contains(t, resp.Header.Get("Content-Security-Policy"), "img-src 'self';")
+	assert.Equal(t, "nosniff", resp.Header.Get("X-Content-Type-Options"))
+	assert.Equal(t, "strict-origin-when-cross-origin", resp.Header.Get("Referrer-Policy"))
 }
 
 func TestRest_subscribersOnly(t *testing.T) {