feat: download ETA on the press bar from measured Soulseek throughput

Show a live "~Xm left" countdown on the download progress bar. The slskd download
loop already polls byte transfer every 3s; measure the actual throughput (EWMA-
smoothed bytes/sec), compute seconds-remaining from the album's total bytes, and
surface it. New nullable Job.downloadEtaSeconds (migration; web entrypoint runs
prisma migrate deploy), threaded through the on_progress callback (optional 2nd arg,
so other adapters/callers are unaffected — they report no ETA). API exposes it;
queue.tsx renders etaLabel() after the track count. Null when unknown (no sample yet
or a source that doesn't report bytes). Worker 330 tests, web 214 tests, both green;
verified live-rendered as "42% · 6/14 tracks · ~4m left".

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

web/src/app/_ui/status.test.ts

@@ -1,5 +1,20 @@
 import { describe, it, expect } from "vitest";
-import { describeJob, timeAgo } from "./status";
+import { describeJob, etaLabel, timeAgo } from "./status";
+
+describe("etaLabel", () => {
+  it("formats seconds, minutes, and hours", () => {
+    expect(etaLabel(45)).toBe("~45s left");
+    expect(etaLabel(240)).toBe("~4m left");
+    expect(etaLabel(3900)).toBe("~1h 5m left");
+  });
+
+  it("returns null for missing or non-positive input", () => {
+    expect(etaLabel(null)).toBeNull();
+    expect(etaLabel(undefined)).toBeNull();
+    expect(etaLabel(0)).toBeNull();
+    expect(etaLabel(-5)).toBeNull();
+  });
+});
 
 describe("describeJob", () => {
   it("maps pipeline states to literal labels + severity kind", () => {