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

The CORS middleware with the `access_control_allow_origin` origin wrapped every route from the router root, so the login and URL generator pages and `/metrics` sent `Access-Control-Allow-Origin` too. It now wraps only `/v1/image/` and `/v1/e/`, which form a `/v1` subrouter so that a browser's preflight `OPTIONS` request still gets its answer; the maintenance mode group moved inside it unchanged. Every other route sends no CORS headers. A new test checks both image routes, a preflight included, and the login and URL generator pages. `README.md` and `config.example.yml` say the setting covers the image routes only.

Unverified by test: `/metrics`, whose middleware registers with the process-wide Prometheus registry.

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`.
 - 2026-10-02 a plain `docker build .` stamps the tag or short commit, not
   `dev` (closes #166): `.dockerignore` lets `.git` into the build context,
   without `.git/config`; with no `VERSION` build argument the `Dockerfile`