Remove the no-op MAINTENANCE_MODE setting (closes #317)

MAINTENANCE_MODE only made the healthcheck report
maintenanceMode: true; no request was ever served differently. It is
removed from the configuration, the healthcheck JSON, the README and
the Settings page, together with the unused Server.MaintenanceMode
method and the healthcheck's Config dependency, which existed only to
read it. A leftover MAINTENANCE_MODE in the environment is not read,
so it is ignored rather than refused.

Model: opus-5-5

internal/handlers/settings_test.go

@@ -62,14 +62,12 @@ func settingsShown(
 func TestSettingsPageShowsLoadedConfiguration(t *testing.T) {
 	t.Parallel()
 
-	// DEBUG and MAINTENANCE_MODE get opposite values, and each of
-	// METRICS_USERNAME, METRICS_PASSWORD and SENTRY_DSN is the only one
-	// of the three set in one of the content tests, so each row is
-	// checked against its own field.
+	// Each of METRICS_USERNAME, METRICS_PASSWORD and SENTRY_DSN is the
+	// only one of the three set in one of the content tests, so each
+	// row is checked against its own field.
 	cfg := &config.Config{
 		DataDir:                t.TempDir(),
 		Debug:                  true,
-		MaintenanceMode:        false,
 		Environment:            config.EnvironmentDev,
 		MetricsUsername:        "scraper",
 		MetricsPassword:        "",
@@ -96,7 +94,6 @@ func TestSettingsPageShowsLoadedConfiguration(t *testing.T) {
 		"BIND_ADDRESS":             "192.0.2.10",
 		"DATA_DIR":                 cfg.DataDir,
 		"DEBUG":                    "true",
-		"MAINTENANCE_MODE":         "false",
 		"METRICS_USERNAME":         "scraper",
 		"METRICS_PASSWORD":         "not set",
 		"SENTRY_DSN":               "not set",