Allowlist no host in the example config (closes #249)

The example config allowlisted github.com and user-images.githubusercontent.com, where anyone can put an image, so a deployment set up as Getting Started says served every image there to anyone without a signature. The example now allowlists nothing and shows one commented entry under example.com. README.md says in Getting Started and in "Running under upaas" that every image on an allowlisted host is served without a signature, so an allowlisted host must be one the operator controls. A new test loads the example config and fails if it allowlists any host.

Model: opus-5-5

TODO.md

@@ -30,6 +30,14 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-15 the example config allowlists no host (closes #249): it allowlisted
+  `github.com` and `user-images.githubusercontent.com`, where anyone can put an
+  image, so a deployment set up as Getting Started says served those images to
+  anyone without a signature, at any size and in any format.
+  `configs/config.example.yml` now leaves `allowlist_hosts` out, with one
+  commented entry under `example.com`. Getting Started and "Running under upaas"
+  in `README.md` say that every image on an allowlisted host is served without a
+  signature, so an allowlisted host must be one the operator controls.
 - 2026-10-13 `fit=outside` no longer makes an output above 8192 on a side
   (closes #257): it scales the image to cover the requested size, and the side
   scaled past that size was held to no maximum, so `1x8192?fit=outside` of a