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

config.example.yml

@@ -12,6 +12,11 @@
 # 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: the image's Docker HEALTHCHECK and upaas read it,
+# and a 503 there would make upaas mark the deploy failed.
 maintenance_mode: false
 
 # Data directory for SQLite database and cache files