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

MAINTENANCE_MODE did nothing but make the healthcheck JSON report maintenanceMode: true; no request was ever served differently, so an operator who set it expecting requests to be refused got nothing. It is removed from the configuration, the startup configuration log line, the healthcheck JSON, the README and the Settings page, together with the uncalled Server.MaintenanceMode method and the healthcheck's dependency on the configuration. A leftover value in an environment is ignored like any other unknown variable.

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",