feat: add nginx proxy for theme injection (cleaner approach)

asciinema/nginx/nginx.conf

@@ -0,0 +1,40 @@
+events {
+    worker_connections 1024;
+}
+
+http {
+    include /etc/nginx/mime.types;
+    default_type application/octet-stream;
+
+    # Map to determine which backend to use based on host
+    map $http_host $backend_port {
+        default 4000;
+        ~^admin\. 4002;
+    }
+
+    server {
+        listen 8080;
+        server_name _;
+
+        # Proxy to actual asciinema server (port depends on subdomain)
+        location / {
+            proxy_pass http://asciinema_backend:$backend_port;
+            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;
+
+            # Inject custom JavaScript into HTML responses
+            sub_filter '</head>' '<script src="/theme/inject.js"></script></head>';
+            sub_filter_once on;
+            sub_filter_types text/html;
+        }
+
+        # Serve theme files directly
+        location /theme/ {
+            alias /theme/;
+            expires 1d;
+            add_header Cache-Control "public, immutable";
+        }
+    }
+}