Send CORS headers only from the image routes (closes #98)

The CORS middleware, with the access_control_allow_origin origin, moved
from the router root onto a /v1 subrouter holding /v1/image/ and /v1/e/.
The login and URL generator pages, /metrics, the health check,
robots.txt and /static/ no longer send Access-Control-Allow-Origin, so
their safety no longer rests on the CORS options chosen for the image
routes.

It is a subrouter rather than a route group because a group's
middleware runs only for a request that matches one of its routes, and
a preflight OPTIONS request matches none. The maintenance mode group
moved inside it unchanged.

README.md and config.example.yml say the setting covers the image
routes only.

Model: opus-5-5

config.example.yml

@@ -103,8 +103,9 @@ upstream_max_response_size: 52428800
 # longer than upstream_fetch_timeout plus 20 seconds.
 downstream_timeout: 60s
 
-# The origin a browser lets read pixa's responses, sent as the CORS
-# Access-Control-Allow-Origin header: "*" (the default) is any site;
+# The origin a browser lets read the responses of the image routes,
+# /v1/image/ and /v1/e/, sent as the CORS Access-Control-Allow-Origin
+# header; no other route sends it. "*" (the default) is any site;
 # otherwise one http or https origin such as https://example.com, whose
 # host is a lowercase host name (letters, digits, hyphens and dots, with a
 # letter in its last part) or an IP address (IPv6 in brackets, in its