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/data/config.ts

@@ -91,6 +91,19 @@ export interface BotConfig {
   idleTimeoutMinutes: number;
   /** Enable uploading and playback of server-stored local audio files. */
   localAudioEnabled: boolean;
+  /**
+   * Enable named save/load of queues (chat + web) AND auto-restore of the live
+   * queue across a restart. Admin-controlled; default false so nothing is
+   * persisted/restored until an operator opts in.
+   */
+  savedQueuesEnabled: boolean;
+  /**
+   * When true, a single-song immediate !play (chat) / play-song (web) inserts
+   * after the current track and jumps to it instead of clearing the queue, so
+   * the rest of the queue survives and continues afterwards. Default false
+   * keeps today's clear-and-play behavior.
+   */
+  playKeepsQueue: boolean;
   // Public base URL used when generating share links (e.g. the bot专属链接).
   // Leave empty to use the browser's current origin. Example:
   //   "https://music.example.com" or "http://1.2.3.4:3000"
@@ -130,6 +143,8 @@ export function getDefaultConfig(): BotConfig {
     autoPauseOnEmpty: false,
     idleTimeoutMinutes: 0,
     localAudioEnabled: true,
+    savedQueuesEnabled: false,
+    playKeepsQueue: false,
     publicUrl: "",
     trustProxy: false,
     guestMode: {
@@ -317,6 +332,12 @@ export function loadConfig(path: string): BotConfig {
         )
       : defaults.enabledProviders;
 
+    // Strict-coerce the two feature flags exactly like spotify.enabled so a
+    // hand-edited / legacy / corrupt config.json can never silently enable
+    // them (`"yes"`, `1`, `null` → false; only a literal `true` enables).
+    const savedQueuesEnabled = partial.savedQueuesEnabled === true;
+    const playKeepsQueue = partial.playKeepsQueue === true;
+
     return {
       ...defaults,
       ...partial,
@@ -325,6 +346,8 @@ export function loadConfig(path: string): BotConfig {
       spotify,
       jellyfin,
       enabledProviders,
+      savedQueuesEnabled,
+      playKeepsQueue,
     };
   }
 }