feat: add robots.txt and sitemap.xml routes for SEO optimization

- Implemented a new route for robots.txt to control crawler access.
- Added a sitemap.xml route to provide search engines with a list of site URLs.
- Included functions for URL normalization and XML escaping to ensure proper formatting.

server/routes/robots.txt.js

@@ -0,0 +1,30 @@
+function normalizeBaseUrl(value) {
+  const raw = String(value || '').trim()
+  if (!raw) return ''
+  return raw.replace(/\/+$/, '')
+}
+
+export default defineEventHandler((event) => {
+  const runtimeConfig = useRuntimeConfig(event)
+  const requestUrl = getRequestURL(event)
+
+  const baseUrl = normalizeBaseUrl(runtimeConfig.public?.baseUrl) || `${requestUrl.protocol}//${requestUrl.host}`
+
+  const lines = [
+    'User-agent: *',
+    'Allow: /',
+    'Disallow: /cms',
+    'Disallow: /cms/',
+    'Disallow: /mitgliederbereich',
+    'Disallow: /mitgliederbereich/',
+    'Disallow: /api/',
+    'Disallow: /login',
+    'Disallow: /registrieren',
+    'Disallow: /passwort-vergessen',
+    'Disallow: /konto-loeschen',
+    `Sitemap: ${baseUrl}/sitemap.xml`
+  ]
+
+  setHeader(event, 'Content-Type', 'text/plain; charset=utf-8')
+  return `${lines.join('\n')}\n`
+})