Answer image requests with 503 in maintenance mode (closes #71)

maintenance_mode was only reported by the health check; every request
was still served. One middleware in routes.go, applied to /v1/image/
and /v1/e/ only, now answers them with 503, a Retry-After of
MaintenanceRetryAfterSeconds and the JSON error body while it is on.
It calls Server.MaintenanceMode(), which had no caller.

The health check stays 200 and reports maintenance_mode: the image's
Docker HEALTHCHECK requests it, a 503 there would make the container
unhealthy, and upaas marks a deploy failed when its container is
unhealthy. The login and URL generator pages and /metrics keep working.
Documented in README.md and config.example.yml.

Model: opus-5-5

TODO.md

@@ -29,6 +29,14 @@ P2: security: referer blacklist
 
 # Completed Steps
 
+- 2026-09-29 maintenance mode refuses image requests (closes #71): while
+  `maintenance_mode` is on, `/v1/image/` and `/v1/e/` answer 503 with a
+  `Retry-After` header and the JSON error body, from one middleware in
+  `internal/server/routes.go`; the health check stays 200 and reports
+  `maintenance_mode`, as the image's Docker `HEALTHCHECK` requests it and upaas
+  marks a deploy failed when its container is unhealthy; the login and URL
+  generator pages and `/metrics` keep working; documented in `README.md` and
+  `config.example.yml`.
 - 2026-09-29 bound concurrent image processing and upstream fetches (closes
   #64): `max_concurrent_processing` (default the number of CPUs pixa can use)
   limits the images decoded and encoded at once, and `upstream_connections`