feat(rust): the live map draws monuments by label, minor labels hidden (PLAN_REDESIGNS §4, D196)

A switch per monument label, a fleet default and a per-server override, on
Admin -> Rust visibility's live map card. Substations, caves, train tunnels,
wells and the other minor labels start hidden; every other label is drawn,
so a monument a game update adds appears. GET /map leaves hidden labels out
of the answer, so the website and the app both lose them.

No schema change: rows are map.marker.<label key> in rust_settings and
rust_map_overrides. No wire change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01E14m6SuuY6i1vASFeGDBeY

server/model/visibility/visibility.db.js

@@ -25,6 +25,22 @@ async function setSetting(key, value, userId = null) {
   )
 }
 
+/**
+ * Every stored setting whose key starts with `prefix`, as `{ setting, value }`
+ * rows: the map's per-label marker switches are one row each (D196).
+ */
+async function listSettings(prefix) {
+  return core.query(
+    `SELECT setting_key AS setting, value FROM ${SETTINGS} WHERE setting_key LIKE CONCAT(?, '%')`,
+    [prefix.replace(/[\\%_]/g, '\\$&')],
+  )
+}
+
+/** Forget one setting, so it follows the built-in default again. */
+async function clearSetting(key) {
+  await core.query(`DELETE FROM ${SETTINGS} WHERE setting_key = ?`, [key])
+}
+
 /** One server's override, `null` for "inherit", or `undefined` when there is no such server. */
 async function getServerPresence(serverId) {
   const rows = await core.query(`SELECT presence_audience AS presence FROM ${SERVERS} WHERE id = ?`, [serverId])
@@ -69,6 +85,8 @@ async function setServerNewsDelivery(serverId, delivery) {
 module.exports = {
   getSetting,
   setSetting,
+  listSettings,
+  clearSetting,
   getServerPresence,
   listServerPresence,
   setServerPresence,