Ship the extension as a signed CRX from our own update server

The dashboard-hosting stack from the previous commit is gone; it was answering
a question nobody asked. What was actually needed is Chrome's real update
channel, which an unpacked extension does not have and cannot be given.

So the extension is now signed into a CRX with an update manifest beside it,
served from the droplet, and installed on the trading machines by policy
(ExtensionInstallForcelist). From there Chrome installs it, updates it on its
own schedule, and puts it back if it goes missing. The manifest gained the
`key` that fixes the extension id, so an unpacked development load keeps the
same identity as the published build.

The self-update check now branches on how it was installed: policy installs ask
Chrome to check the update server now rather than waiting out its five-hour
cycle, and unpacked loads keep the reload-from-disk path. chrome.management
.getSelf() tells the two apart and needs no permission to do it.

The signing key lives at /opt/autofirmer-updates/key.pem on the droplet and
nowhere else. It is the extension's identity: a different key is a different
extension, and no browser would take it as an update.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

deploy/nginx/ext.conf.template

@@ -0,0 +1,45 @@
+# The extension update server — rendered into the autotrader stack's nginx/conf.d
+# by deploy.sh. Edit the template, not the rendered file.
+server {
+    listen 80;
+    listen [::]:80;
+    server_name ${EXT_HOST};
+
+    location /.well-known/acme-challenge/ {
+        root /var/www/certbot;
+    }
+
+    location / {
+        return 301 https://$host$request_uri;
+    }
+}
+
+server {
+    listen 443 ssl;
+    listen [::]:443 ssl;
+    http2 on;
+    server_name ${EXT_HOST};
+
+    ssl_certificate     /etc/letsencrypt/live/${EXT_HOST}/fullchain.pem;
+    ssl_certificate_key /etc/letsencrypt/live/${EXT_HOST}/privkey.pem;
+    ssl_protocols       TLSv1.2 TLSv1.3;
+    ssl_prefer_server_ciphers off;
+    ssl_session_cache   shared:SSL:10m;
+    ssl_session_timeout 1d;
+
+    add_header Strict-Transport-Security "max-age=31536000" always;
+    add_header X-Content-Type-Options "nosniff" always;
+
+    # Two public files: the update manifest and the signed CRX. Both are meant to
+    # be readable by anyone — Chrome fetches them with no cookies and no way to
+    # authenticate, which is why there is no auth here. The CRX is signed, and
+    # carries no credentials; the private key stays on the droplet.
+    location / {
+        proxy_pass         http://autofirmer-updates/;
+        proxy_http_version 1.1;
+        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;
+    }
+}