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

config.example.yml

@@ -16,6 +16,12 @@
 # Server settings
 port: 8080
 debug: false
+
+# While true, the image routes (/v1/image/ and /v1/e/) answer every request
+# with 503 and a Retry-After header. The health check keeps answering 200 and
+# reports maintenance_mode as true. It stays 200 because 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.
 maintenance_mode: false
 
 # Data directory for SQLite database and cache files