Firmware CI releases + Gitea auto-update in the server

New Gitea Actions workflow builds both board variants and publishes
them as release assets whenever firmware/version.txt is bumped. The
server can now poll that repo's releases (next to the existing manual
upload) and either surface an "Update frame" button or, with
"Automatically apply updates" checked, stage the new build itself --
the frame still only updates on its own next wake either way.

server/docker-compose.yml.example

@@ -17,4 +17,9 @@ services:
       # (Access Token field) so it's sent on every device request and gets
       # embedded automatically in the manage-menu/share QR codes.
       - MANAGEMENT_TOKEN=changeme
+      # Optional: only needed if the Gitea repo configured in the web UI's
+      # "Firmware Gitea repo URL" field is private. A read-only PAT is
+      # enough -- it's sent to that repo's releases API/asset downloads
+      # only, never exposed via the web UI or any API response.
+      - GITEA_FIRMWARE_TOKEN=your-gitea-pat-here
     restart: unless-stopped