feat(#119): save/load queues + live-queue persistence + playKeepsQueue (backend)

Add three default-off capabilities that stop the play queue from being lost,
all gated behind admin/independent toggles so existing behavior is unchanged
until an operator opts in:

- Named save/load of queues (Feature 1): new saved_queues table (per-user +
  reserved __shared__ owner, capped at 50 queues / 1000 songs, JSON song blob
  that degrades to empty on corruption); /api/saved-queues router (list/save/
  load/delete with ownership 404s, inert 403 when disabled); chat commands
  !save / !load [-a] / !queues; BotInstance.loadSavedQueue (replace/append).
- Auto-restore live queue across restart (Feature 2): PlayQueue.snapshot/restore,
  queue_state table (one row per bot), a debounced snapshot writer driven off
  stateChange, and restore+resume on connect. Cancels the pending snapshot on
  disconnect so a stale write can't wipe the row a restart must restore.
- playKeepsQueue (Feature 3): BotInstance.playSingleSong funnels chat !play and
  the web /play-song route through one place; when enabled a single-song play
  inserts-after-current and jumps instead of clearing the queue.

Config gains savedQueuesEnabled + playKeepsQueue (both default false, strict-
coerced on load like spotify.enabled); the settings API round-trips them.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/web/api/bot.ts

@@ -72,6 +72,8 @@ export function createBotRouter(
       idleTimeoutMinutes: config.idleTimeoutMinutes ?? 0,
       autoPauseOnEmpty: config.autoPauseOnEmpty,
       localAudioEnabled: config.localAudioEnabled,
+      savedQueuesEnabled: config.savedQueuesEnabled,
+      playKeepsQueue: config.playKeepsQueue,
       adminGroups: config.adminGroups ?? [],
       guestMode: config.guestMode,
       spotify: maskedSpotify(),
@@ -98,6 +100,16 @@ export function createBotRouter(
     if (hasAutoPause) config.autoPauseOnEmpty = autoPauseOnEmpty;
     if (hasLocalAudioEnabled) config.localAudioEnabled = localAudioEnabled;
 
+    // Saved-queues + play-keeps-queue toggles (default off). Both read live from
+    // config by BotInstance / the saved-queues router, so no per-bot push needed;
+    // only a literal boolean mutates the stored value (junk is ignored).
+    if (typeof req.body.savedQueuesEnabled === "boolean") {
+      config.savedQueuesEnabled = req.body.savedQueuesEnabled;
+    }
+    if (typeof req.body.playKeepsQueue === "boolean") {
+      config.playKeepsQueue = req.body.playKeepsQueue;
+    }
+
     const hasGuestMode = guestMode !== undefined && guestMode !== null && typeof guestMode === "object";
     if (hasGuestMode) {
       const gm = config.guestMode;
@@ -214,6 +226,8 @@ export function createBotRouter(
       idleTimeoutMinutes: config.idleTimeoutMinutes ?? 0,
       autoPauseOnEmpty: config.autoPauseOnEmpty,
       localAudioEnabled: config.localAudioEnabled,
+      savedQueuesEnabled: config.savedQueuesEnabled,
+      playKeepsQueue: config.playKeepsQueue,
       adminGroups: config.adminGroups ?? [],
       guestMode: config.guestMode,
       spotify: maskedSpotify(),