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.test.ts

@@ -494,4 +494,28 @@ describe("loadConfig error handling", () => {
       expect(readFileSync(join(dir, backups[0]), "utf-8")).toBe(content);
     }
   });
+
+  it("defaults savedQueuesEnabled and playKeepsQueue to false", () => {
+    const c = getDefaultConfig();
+    expect(c.savedQueuesEnabled).toBe(false);
+    expect(c.playKeepsQueue).toBe(false);
+  });
+
+  it("coerces non-boolean savedQueues/playKeepsQueue values to false on load", () => {
+    const dir = makeTmpDir();
+    const path = join(dir, "config.json");
+    writeFileSync(path, JSON.stringify({ savedQueuesEnabled: "yes", playKeepsQueue: 1 }));
+    const c = loadConfig(path);
+    expect(c.savedQueuesEnabled).toBe(false);
+    expect(c.playKeepsQueue).toBe(false);
+  });
+
+  it("preserves savedQueues/playKeepsQueue true when explicitly enabled", () => {
+    const dir = makeTmpDir();
+    const path = join(dir, "config.json");
+    writeFileSync(path, JSON.stringify({ savedQueuesEnabled: true, playKeepsQueue: true }));
+    const c = loadConfig(path);
+    expect(c.savedQueuesEnabled).toBe(true);
+    expect(c.playKeepsQueue).toBe(true);
+  });
 });