feat: track download progress on Job (migration + worker poller + API)

Add Job.downloadProgress (0.0-1.0), derived from a worker background
poller that counts audio files landing in staging / expected track
count, since streamrip only reports 0/1. No UI yet (slice B).

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

web/src/app/api/requests/route.ts

@@ -63,6 +63,7 @@ export async function GET() {
               error: r.job.error,
               claimedAt: r.job.claimedAt,
               updatedAt: r.job.updatedAt,
+              downloadProgress: r.job.downloadProgress,
               candidateCount: cands.length,
               chosen: chosen ? { source: chosen.source, format: chosen.format, trackCount: chosen.trackCount } : null,
             };