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.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 " +