feat(discovery): web ListenBrainz similar-artists client

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/lib/listenbrainz.test.ts

@@ -0,0 +1,22 @@
+import { describe, it, expect, vi, afterEach } from "vitest";
+import { similarArtists } from "./listenbrainz";
+
+afterEach(() => vi.restoreAllMocks());
+
+describe("similarArtists", () => {
+  it("parses rows and drops the seed itself", async () => {
+    vi.spyOn(global, "fetch").mockResolvedValue(
+      new Response(JSON.stringify([
+        { artist_mbid: "seed", name: "Seed", score: 100 },
+        { artist_mbid: "c1", name: "Cand", score: 42 },
+      ]), { status: 200 }),
+    );
+    const out = await similarArtists("seed");
+    expect(out).toEqual([{ mbid: "c1", name: "Cand", score: 42 }]);
+  });
+
+  it("throws on a non-2xx response", async () => {
+    vi.spyOn(global, "fetch").mockResolvedValue(new Response("nope", { status: 500 }));
+    await expect(similarArtists("seed")).rejects.toThrow();
+  });
+});

web/src/lib/listenbrainz.ts

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+const LB_BASE = process.env.LISTENBRAINZ_URL ?? "https://labs.api.listenbrainz.org";
+// Keep in sync with the worker's _listenbrainz.py default algorithm.
+const ALGORITHM =
+  "session_based_days_7500_session_300_contribution_5_threshold_15_limit_50_filter_True_skip_30";
+
+export type SimilarArtist = { mbid: string; name: string; score: number };
+
+export async function similarArtists(artistMbid: string): Promise<SimilarArtist[]> {
+  const url =
+    `${LB_BASE}/similar-artists/json?artist_mbids=${encodeURIComponent(artistMbid)}` +
+    `&algorithm=${encodeURIComponent(ALGORITHM)}`;
+  const res = await fetch(url, { headers: { Accept: "application/json" } });
+  if (!res.ok) throw new Error(`ListenBrainz request failed: ${res.status}`);
+  const rows: any[] = (await res.json()) ?? [];
+  return rows
+    .filter((r) => r.artist_mbid && r.artist_mbid !== artistMbid)
+    .map((r) => ({ mbid: r.artist_mbid, name: r.name ?? r.comment ?? "", score: Number(r.score ?? 0) }));
+}