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

configs/config.example.yml 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. The example 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.

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