feat(search): server-side offset pagination through providers + /search route (#115 backend)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/web/api/music.test.ts

@@ -0,0 +1,49 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import express from "express";
+import request from "supertest";
+import pino from "pino";
+import type { MusicProvider, SearchResult } from "../../music/provider.js";
+import { createMusicRouter } from "./music.js";
+
+function fakeProvider(platform: MusicProvider["platform"]): MusicProvider {
+  const empty: SearchResult = { songs: [], albums: [], playlists: [] };
+  return {
+    platform,
+    search: vi.fn().mockResolvedValue(empty),
+  } as unknown as MusicProvider;
+}
+
+describe("music router GET /search offset pagination", () => {
+  let app: express.Express;
+  let netease: MusicProvider;
+
+  beforeEach(() => {
+    netease = fakeProvider("netease");
+    const router = createMusicRouter(
+      netease,
+      fakeProvider("qq"),
+      fakeProvider("bilibili"),
+      pino({ level: "silent" })
+    );
+    app = express();
+    app.use("/api/music", router);
+  });
+
+  it("parses offset and passes it as the 3rd arg to provider.search", async () => {
+    const res = await request(app).get("/api/music/search?q=hello&limit=20&offset=20");
+    expect(res.status).toBe(200);
+    expect(netease.search).toHaveBeenCalledWith("hello", 20, 20);
+  });
+
+  it("defaults a missing offset to 0", async () => {
+    const res = await request(app).get("/api/music/search?q=hello&limit=20");
+    expect(res.status).toBe(200);
+    expect(netease.search).toHaveBeenCalledWith("hello", 20, 0);
+  });
+
+  it("clamps a negative offset to 0", async () => {
+    const res = await request(app).get("/api/music/search?q=hello&limit=20&offset=-5");
+    expect(res.status).toBe(200);
+    expect(netease.search).toHaveBeenCalledWith("hello", 20, 0);
+  });
+});