feat: configure nginx proxy and response headers to support SSE for label print agent

paperless-frontend/nginx.conf

@@ -4,6 +4,21 @@ server {
     root /usr/share/nginx/html;
     index index.html;
 
+    # ── SSE-Endpunkt (kein Puffern, langer Timeout) ────────────
+    location /api/label-print-agent/events {
+        proxy_pass http://backend:3100/api/label-print-agent/events;
+        proxy_http_version 1.1;
+        proxy_set_header Connection '';
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
+        proxy_buffering off;
+        proxy_cache off;
+        proxy_read_timeout 24h;
+        chunked_transfer_encoding on;
+    }
+
     # ── API Reverse Proxy → Backend-Container ──────────────────
     # Im Docker-Netzwerk ist "backend" der Service-Name aus docker-compose.
     location /api/ {