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/prisma/schema.prisma

@@ -58,6 +58,9 @@ model Job {
   createdAt        DateTime    @default(now())
   updatedAt        DateTime    @updatedAt
   downloadProgress Float       @default(0)
+  // Estimated seconds until the current download finishes, from measured throughput (slskd byte
+  // transfer rate). Null when unknown (no speed sample yet, or a source that doesn't report bytes).
+  downloadEtaSeconds Int?
   paused           Boolean     @default(false)
   candidates       Candidate[]
 }