docs(fpagent): document the one-line installer and its key

.env.example

@@ -1,6 +1,6 @@
-# Image Cropper service credentials — the ONLY environment variables this app
-# uses. Everything else (URLs, port, domain catalog) is hardcoded in source.
-# Without these the rest of the app works and /cropper answers 503.
+# Image Cropper service credentials. Everything else (URLs, port, domain
+# catalog) is hardcoded in source. Without these the rest of the app works and
+# /cropper answers 503.
 #
 # This repository uses dotenvx-encrypted env files (.env, .env.production);
 # the private keys live in .env.keys (gitignored). To set up or change a value:
@@ -16,3 +16,9 @@ AZURE_STORAGE_CONNECTION_STRING=
 NODERED_HTTP_USERNAME=
 NODERED_HTTP_PASSWORD=
 CARVEKIT_API_KEY=
+
+# Token the server uses to read the private fpagent releases from the forge
+# (https://sources.yrka.app). It stays server-side: /fpagent/releases/* proxies
+# the bundles for the public installer at /fpagent/install.sh. Without it the
+# installer answers 503.
+GITEA_RELEASE_INSTALL_KEY=