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, since the
image's Docker HEALTHCHECK and upaas read it; the login and URL
generator pages and /metrics keep working. Documented in README.md and
config.example.yml.

Model: opus-5-5

README.md

@@ -239,7 +239,7 @@ variables set by the file's `env:` section are checked the same way.
 | `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                            |
 | `PIXA_DEBUG`                         | `debug`                         | Debug logging and plain-HTTP local development; default `false`              |
-| `PIXA_MAINTENANCE_MODE`              | `maintenance_mode`              | Maintenance flag reported by the health check; default `false`               |
+| `PIXA_MAINTENANCE_MODE`              | `maintenance_mode`              | Answer image requests with 503; the health check stays 200; default `false`  |
 
 Key settings in more detail:
 
@@ -276,6 +276,12 @@ Key settings in more detail:
 - `cache_max_bytes` — disk cache size limit in bytes; `0` disables the
   disk cache entirely; omitted defaults to 75% of the free space on
   the filesystem containing `<state_dir>/cache/` (minimum 500 MiB)
+- `maintenance_mode` — while `true`, the image routes (`/v1/image/` and
+  `/v1/e/`) answer every request with 503, a `Retry-After` header and a JSON
+  error body. The health check (`/.well-known/healthcheck.json`) still answers
+  200 and reports `"maintenance_mode": true`: the image's Docker `HEALTHCHECK`
+  and upaas both read it, and a 503 there would make upaas mark the deploy
+  failed. The login and URL generator pages and `/metrics` keep working
 
 See `config.example.yml` for all options with defaults.