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

TODO.md

@@ -29,6 +29,12 @@ P2: security: referer blacklist
 
 # Completed Steps
 
+- 2026-09-29 only the image routes send CORS headers (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/`, where it
+  still answers a preflight `OPTIONS` request; the login and URL generator
+  pages, `/metrics` and the other routes send no `Access-Control-Allow-Origin`;
+  documented in `README.md` and `config.example.yml`.
 - 2026-09-29 the container makes `/var/lib/pixa` usable by itself (closes
   #159): `deploy/docker-entrypoint.sh` creates the directory if it is missing,
   gives the directory and everything in it to `pixad` when the directory or one