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

README.md

@@ -238,7 +238,7 @@ variables set by the file's `env:` section are checked the same way.
 | `PIXA_UPSTREAM_FETCH_TIMEOUT`        | `upstream_fetch_timeout`        | Time allowed for one fetch from an upstream host; default `30s`              |
 | `PIXA_UPSTREAM_MAX_RESPONSE_SIZE`    | `upstream_max_response_size`    | Largest upstream response accepted, in bytes; default 50 MiB                 |
 | `PIXA_DOWNSTREAM_TIMEOUT`            | `downstream_timeout`            | Time allowed for answering one client request; default `60s`                 |
-| `PIXA_ACCESS_CONTROL_ALLOW_ORIGIN`   | `access_control_allow_origin`   | CORS origin allowed to read responses: `*` or one origin; default `*`        |
+| `PIXA_ACCESS_CONTROL_ALLOW_ORIGIN`   | `access_control_allow_origin`   | CORS origin allowed to read image responses: `*` or one origin; default `*`  |
 | `PIXA_METRICS_USERNAME`              | `metrics.username`              | Username for `/metrics`, which is served only when both are set              |
 | `PIXA_METRICS_PASSWORD`              | `metrics.password`              | Password for `/metrics`; set together with the username                      |
 | `PIXA_SENTRY_DSN`                    | `sentry_dsn`                    | Sentry DSN for error reporting; empty disables it                            |
@@ -247,8 +247,9 @@ variables set by the file's `env:` section are checked the same way.
 
 Key settings in more detail:
 
-- `access_control_allow_origin` — the origin a browser lets read pixa's
-  responses, sent as the CORS `Access-Control-Allow-Origin` header: `*`, the
+- `access_control_allow_origin` — 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