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.test.ts

@@ -325,6 +325,38 @@ describe("bot router /settings", () => {
     expect(res.status).toBe(200);
     expect(config.spotify).toEqual(before);
   });
+
+  it("GET /settings echoes savedQueuesEnabled + playKeepsQueue (default false)", async () => {
+    const res = await request(app).get("/api/bot/settings").set("Cookie", cookie);
+    expect(res.status).toBe(200);
+    expect(res.body.savedQueuesEnabled).toBe(false);
+    expect(res.body.playKeepsQueue).toBe(false);
+  });
+
+  it("POST /settings persists savedQueuesEnabled and playKeepsQueue", async () => {
+    const res = await request(app)
+      .post("/api/bot/settings")
+      .set("Cookie", cookie)
+      .send({ savedQueuesEnabled: true, playKeepsQueue: true });
+    expect(res.status).toBe(200);
+    expect(res.body.savedQueuesEnabled).toBe(true);
+    expect(res.body.playKeepsQueue).toBe(true);
+    expect(config.savedQueuesEnabled).toBe(true);
+    expect(config.playKeepsQueue).toBe(true);
+
+    const get = await request(app).get("/api/bot/settings").set("Cookie", cookie);
+    expect(get.body.savedQueuesEnabled).toBe(true);
+    expect(get.body.playKeepsQueue).toBe(true);
+  });
+
+  it("POST /settings ignores non-boolean savedQueuesEnabled without 400", async () => {
+    const res = await request(app)
+      .post("/api/bot/settings")
+      .set("Cookie", cookie)
+      .send({ savedQueuesEnabled: "nope" });
+    expect(res.status).toBe(200);
+    expect(config.savedQueuesEnabled).toBe(false); // unchanged from default
+  });
 });
 
 // Whole-branch I2: saving a Client ID in Settings must re-configure the single