fix(karrot): fix nginx DNS resolver for Kubernetes and add README

The karrot-frontend nginx template hardcodes Docker's DNS resolver
(127.0.0.11) which doesn't exist in Kubernetes. Added a ConfigMap to
override the template, removing the Docker DNS resolver by using a
direct proxy_pass with no nginx variable (which would force runtime
resolution). Also adds a README with usage instructions.

Documents lessons learned in ADDING-APPS-NOTES.md:
- Note 24: nginx Docker DNS resolver doesn't work in Kubernetes; any
  nginx variable in proxy_pass (including $request_uri) forces runtime
  DNS resolution requiring a resolver directive
- Note 25: ConfigMap changes don't restart pods; subPath mounts never
  auto-update; always follow with kubectl rollout restart
- Note 26: includeSelectors mismatch affects every deployment in an
  app — check all endpoints, not just the web-facing one

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

karrot/versions/1/nginx-template-configmap.yaml

@@ -0,0 +1,116 @@
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: karrot-nginx-template
+  namespace: {{ .namespace }}
+data:
+  default.conf.template: |
+    map $http_upgrade $connection_upgrade {
+        default upgrade;
+        '' close;
+    }
+
+    client_max_body_size ${FILE_UPLOAD_MAX_SIZE};
+
+    server {
+
+        listen ${LISTEN};
+
+        set_real_ip_from 10.0.0.0/8;
+        set_real_ip_from 172.16.0.0/12;
+        set_real_ip_from 192.168.0.0/16;
+        real_ip_header X-Forwarded-For;
+
+        root /usr/share/nginx/html;
+
+        location /assets {
+          expires max;
+        }
+
+        location /css {
+          expires max;
+        }
+
+        location /js {
+          expires max;
+        }
+
+        location /img {
+          expires max;
+        }
+
+        location /fonts {
+          expires max;
+        }
+
+        location / {
+
+            try_files $uri /index.html;
+
+            add_header Strict-Transport-Security "max-age=63072000; includeSubdomains; preload";
+            add_header X-Content-Type-Options nosniff;
+
+            add_header Last-Modified $date_gmt;
+            add_header Cache-Control 'no-store, no-cache, must-revalidate, proxy-revalidate, max-age=0';
+
+            add_header Content-Security-Policy "default-src 'self'; script-src 'self' 'unsafe-eval' 'unsafe-inline'; connect-src 'self' https://nominatim.openstreetmap.org https://sentry.io https://*.ingest.sentry.io ${CSP_CONNECT_SRC} blob:; style-src 'self' 'unsafe-inline'; font-src 'self' data:; img-src 'self' https: data: blob:;";
+
+            if_modified_since off;
+            expires off;
+            etag off;
+        }
+
+        location /bundlesize.html {
+            add_header Content-Security-Policy "default-src 'self' 'unsafe-inline';";
+            add_header Cache-Control 'no-store, no-cache, must-revalidate, proxy-revalidate, max-age=0';
+        }
+
+        location ~ ^\/(api(\-auth)?|docs|silk|static)\/ {
+            proxy_set_header X-Real-IP $remote_addr;
+            proxy_set_header X-Forwarded-For $remote_addr;
+            proxy_set_header Host $http_host;
+
+            proxy_pass http://${BACKEND};
+
+            proxy_redirect off;
+            proxy_http_version 1.1;
+            proxy_set_header Upgrade $http_upgrade;
+            proxy_set_header Connection $connection_upgrade;
+            proxy_set_header Sec-WebSocket-Extensions $http_sec_websocket_extensions;
+            proxy_set_header Sec-WebSocket-Key $http_sec_websocket_key;
+            proxy_set_header Sec-WebSocket-Protocol $http_sec_websocket_protocol;
+            proxy_set_header Sec-WebSocket-Version $http_sec_websocket_version;
+        }
+
+        location /media/ {
+            alias ${FILE_UPLOAD_DIR};
+            expires max;
+        }
+
+        location /media/tmp/ {
+            deny all;
+            return 404;
+        }
+
+        location ~ /media/conversation_message_attachment_(files|previews|thumbnails)/ {
+            deny all;
+            return 404;
+        }
+
+        location /uploads/ {
+            internal;
+            alias ${FILE_UPLOAD_DIR};
+            etag on;
+        }
+
+        location /community_proxy/ {
+            proxy_pass https://community.karrot.world/;
+        }
+
+        error_page 503 @maintenance;
+
+        location @maintenance {
+          try_files /maintenance.html =503;
+        }
+
+    }