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

configs/config.example.yml

@@ -45,15 +45,14 @@ state_dir: ./data
 signing_key: "CHANGE_ME_generate_with_openssl_rand_base64_32"
 
 # Hosts that don't require signatures (default: none)
+# Every image on an allowlisted host is served to anyone without a
+# signature, at any size and in any format, so list only hosts you
+# control, where nobody else can put images.
 # Use "." prefix for wildcard subdomain matching (e.g., ".example.com" matches "cdn.example.com")
 # An entry that is neither a host name nor an IP address (IPv6 without
 # brackets), such as one with a port or a "*." wildcard, aborts startup.
-allowlist_hosts:
-  - s3.sneak.cloud
-  - static.sneak.cloud
-  - sneak.berlin
-  - github.com
-  - user-images.githubusercontent.com
+# allowlist_hosts:
+#   - images.example.com
 
 # Hosts whose pages may not show pixa's images, written as for
 # allowlist_hosts. A request to /v1/image/ or /v1/e/ whose Referer header