feat(fm): let each web user link their own NetEase account for personal FM (#164)

With several people sharing one bot, personal FM always followed the one
account the bot was logged in with. Each signed-in (non-guest) web user
can now scan a QR code under Settings → 账户 to link their own NetEase
account; FM they start from the WebUI then comes from their account.

- user_music_cookies table (per user + platform, dropped with the user).
- NeteaseProvider.pollQrLogin returns the cookie without storing it, so
  a personal login can never replace the bot's shared account;
  checkQrCodeStatus is now built on it. withCookie gives a view bound to
  another account.
- /api/me/music/netease: status / qrcode / qrcode/status / unlink, acting
  only on req.user. The cookie never leaves the server.
- POST /api/player/:botId/fm uses the caller's linked account for
  NetEase. Songs still resolve through the shared provider when played.

TeamSpeak chat !fm keeps using the shared account: chat users are not
tied to web accounts.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/music/netease.test.ts

@@ -139,3 +139,48 @@ describe("NeteaseProvider.search pagination", () => {
     expect(callByType(get, 10).offset).toBe(0);
   });
 });
+
+describe("NeteaseProvider per-user login (#164)", () => {
+  function withGet(p: NeteaseProvider, impl: (path: string, cfg: any) => any) {
+    const get = vi.fn(async (path: string, cfg: any) => ({ data: impl(path, cfg) }));
+    (p as any).api = { get, defaults: { baseURL: "http://127.0.0.1:3001" } };
+    return get;
+  }
+
+  it("pollQrLogin returns the cookie without touching the shared account", async () => {
+    const p = new NeteaseProvider("http://127.0.0.1:3001");
+    p.setCookie("MUSIC_U=shared");
+    withGet(p, () => ({ code: 803, cookie: "MUSIC_U=personal" }));
+    expect(await p.pollQrLogin("k")).toEqual({ status: "confirmed", cookie: "MUSIC_U=personal" });
+    expect(p.getCookie()).toBe("MUSIC_U=shared");
+  });
+
+  it("pollQrLogin maps the waiting / scanned / expired codes", async () => {
+    const p = new NeteaseProvider("http://127.0.0.1:3001");
+    let code = 801;
+    withGet(p, () => ({ code }));
+    expect(await p.pollQrLogin("k")).toEqual({ status: "waiting" });
+    code = 802;
+    expect(await p.pollQrLogin("k")).toEqual({ status: "scanned" });
+    code = 800;
+    expect(await p.pollQrLogin("k")).toEqual({ status: "expired" });
+  });
+
+  it("checkQrCodeStatus still stores the cookie on the shared provider (admin login)", async () => {
+    const p = new NeteaseProvider("http://127.0.0.1:3001");
+    withGet(p, () => ({ code: 803, cookie: "MUSIC_U=admin" }));
+    expect(await p.checkQrCodeStatus("k")).toBe("confirmed");
+    expect(p.getCookie()).toBe("MUSIC_U=admin");
+  });
+
+  it("withCookie gives a view that fetches FM with the other account's cookie", async () => {
+    const p = new NeteaseProvider("http://127.0.0.1:3001");
+    p.setCookie("MUSIC_U=shared");
+    const personal = p.withCookie("MUSIC_U=personal");
+    const get = withGet(personal, () => ({ data: [] }));
+    await personal.getPersonalFm();
+    expect(get.mock.calls[0][1].params.cookie).toBe("MUSIC_U=personal");
+    expect(p.getCookie()).toBe("MUSIC_U=shared");
+    expect(personal.platform).toBe("netease");
+  });
+});