feat: migrate middleware.ts to proxy.ts

- Rename middleware.ts to proxy.ts using Next.js codemod
- Update function name from 'middleware' to 'proxy'
- Update documentation in I18N.md to reflect proxy.ts usage
- Maintain all existing functionality (i18n routing and security headers)

proxy.ts

@@ -0,0 +1,43 @@
+import createMiddleware from 'next-intl/middleware';
+import type { NextRequest } from 'next/server';
+
+const i18nMiddleware = createMiddleware({
+    locales: ['de', 'en'],
+    defaultLocale: 'de',
+    // Wir nutzen überall Locale-Präfixe (`/de`, `/en`)
+    localePrefix: 'always'
+});
+
+export default function proxy(request: NextRequest) {
+    // 1. i18n-Routing
+    const response = i18nMiddleware(request);
+
+    // 2. Security-Header ergänzen
+    const headers = response.headers;
+
+    headers.set('X-Frame-Options', 'SAMEORIGIN');
+    headers.set('X-XSS-Protection', '1; mode=block');
+    headers.set('X-Content-Type-Options', 'nosniff');
+    headers.set('Referrer-Policy', 'strict-origin-when-cross-origin');
+    headers.set('Permissions-Policy', 'camera=(), microphone=(), geolocation=()');
+
+    const csp = [
+        "default-src 'self'",
+        "script-src 'self' 'unsafe-inline' 'unsafe-eval' https://plausible.elpatron.me",
+        "style-src 'self' 'unsafe-inline'",
+        "img-src 'self' data: blob:",
+        "font-src 'self' data:",
+        "connect-src 'self' https://openrouter.ai https://gotify.example.com https://plausible.elpatron.me",
+        "media-src 'self' blob:",
+        "frame-ancestors 'self'",
+    ].join('; ');
+    headers.set('Content-Security-Policy', csp);
+
+    return response;
+}
+
+export const config = {
+    // Empfohlener Matcher aus der next-intl Doku:
+    // alle Routen außer _next, API und statischen Dateien
+    matcher: ['/((?!api|_next|.*\\..*).*)']
+};