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.go

@@ -53,13 +53,6 @@ func settingRows(cfg *config.Config) []settingRow {
 		},
 		{"DATA_DIR", "Directory for all SQLite databases", cfg.DataDir},
 		{"DEBUG", "Enable debug logging", strconv.FormatBool(cfg.Debug)},
-		{
-			"MAINTENANCE_MODE",
-			"Report maintenanceMode: true in the healthcheck JSON. " +
-				"It does not change how any request is served — no " +
-				"maintenance page exists",
-			strconv.FormatBool(cfg.MaintenanceMode),
-		},
 		{
 			"METRICS_USERNAME",
 			"Basic auth username for /metrics. Must be set together " +