fix(nginx): serve .txt as UTF-8 and make AI-discovery files reachable

llms.txt and robots.txt were served without a charset, so browsers
decoded the valid UTF-8 as Windows-1252 and showed mojibake (â€", für).

- Add `charset utf-8;` (+ charset_types) so text responses carry
  `; charset=utf-8`.
- Add an explicit `location ~* \.txt$` that serves the file as plain
  text, returns 404 instead of falling back to the SPA index.html, and
  sets `Access-Control-Allow-Origin: *` so any AI crawler/tool can fetch
  llms.txt / llms-full.txt cross-origin.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docker/nginx.conf

@@ -11,6 +11,22 @@ server {
   root /usr/share/nginx/html;
   index index.html;
 
+  # Serve text responses as UTF-8. Without this nginx omits the charset and
+  # browsers fall back to a locale guess (Windows-1252 on DE systems), which
+  # turns valid UTF-8 in llms.txt / robots.txt into mojibake (â€", für).
+  charset utf-8;
+  charset_types text/plain text/css text/xml application/javascript application/json image/svg+xml;
+
+  # AI-discovery files (llms.txt, llms-full.txt, robots.txt): always served as
+  # UTF-8 plain text, never swallowed by the SPA fallback, and readable
+  # cross-origin so any AI crawler or tool can fetch them.
+  location ~* \.txt$ {
+    try_files $uri =404;
+    default_type text/plain;
+    add_header Access-Control-Allow-Origin "*" always;
+    add_header Cache-Control "public, max-age=3600" always;
+  }
+
   # SPA fallback so /admin, /impressum, /datenschutz hit React Router.
   location / {
     try_files $uri $uri/ /index.html;