feat(downloads): clear multi-phase download progress + transient-error retries

The progress bar looked broken (95% -> 5% -> recount) because yt-dlp downloads the video and
audio streams separately (each 0->100%) then merges — with no indication of which step is
running. Now:
- worker labels the phase from the stream codecs (video / audio) and via a postprocessor hook
  (merging / processing), so the user sees what's happening
- DownloadCenter shows the phase as the status and renders an indeterminate pulse (no bogus %)
  during merge/processing; phase i18n en/hu/de
- yt-dlp retries (3) + fragment_retries (5) so transient network blips self-heal instead of
  failing the job (seen once as a DNS 'No address associated with hostname' error)

Verified: phase transitions queued -> video -> processing -> done.

backend/app/downloads/formats.py

@@ -91,6 +91,10 @@ def build_ydl_opts(spec: dict, outtmpl: str, progress_hook) -> dict:
         "writethumbnail": True,  # kept for the poster.jpg sidecar (and embed, if requested)
         "progress_hooks": [progress_hook],
         "postprocessors": [],
+        # Self-heal transient network blips instead of failing the whole job.
+        "retries": 3,
+        "fragment_retries": 5,
+        "retry_sleep": {"http": 2, "fragment": 2},
     }
 
     if s["mode"] == "a":

backend/app/worker.py

@@ -219,6 +219,18 @@ def _make_progress_hook(job_id: int, asset_id: int):
         if now - state["last_db"] < 1.0:
             return
         state["last_db"] = now
+        # yt-dlp downloads the video and audio streams SEPARATELY (each 0→100%), then merges.
+        # Label the phase so the user understands the bar resetting between streams instead of
+        # seeing a mysterious 95%→5% jump.
+        info = d.get("info_dict") or {}
+        vcodec = info.get("vcodec") or "none"
+        acodec = info.get("acodec") or "none"
+        if vcodec != "none" and acodec == "none":
+            phase = "video"
+        elif acodec != "none" and vcodec == "none":
+            phase = "audio"
+        else:
+            phase = "media"
         total = d.get("total_bytes") or d.get("total_bytes_estimate") or 0
         done = d.get("downloaded_bytes") or 0
         pct = int(done * 100 / total) if total else 0
@@ -227,12 +239,25 @@ def _make_progress_hook(job_id: int, asset_id: int):
             progress=min(pct, 99),
             speed_bps=int(d["speed"]) if d.get("speed") else None,
             eta_s=int(d["eta"]) if d.get("eta") else None,
-            phase="downloading",
+            phase=phase,
         )
 
     return hook
 
 
+def _make_pp_hook(job_id: int):
+    """Postprocessor hook: surface the ffmpeg merge/convert/extract step so the bar shows a
+    'Merging/Processing' label instead of freezing at 100% after the streams finish."""
+
+    def hook(d: dict) -> None:
+        if d.get("status") in ("started", "processing"):
+            pp = d.get("postprocessor") or ""
+            phase = "merging" if "Merg" in pp else "processing"
+            _set_job(job_id, phase=phase, speed_bps=None, eta_s=None)
+
+    return hook
+
+
 def _staging_dir(asset_id: int) -> Path:
     return Path(settings.download_root) / ".staging" / f"asset-{asset_id}"
 
@@ -253,6 +278,7 @@ def _download(job_id: int, asset_id: int, source_kind: str, source_ref: str, spe
 
         outtmpl = str(staging / "%(id)s.%(ext)s")
         opts = formats.build_ydl_opts(spec, outtmpl, _make_progress_hook(job_id, asset_id))
+        opts["postprocessor_hooks"] = [_make_pp_hook(job_id)]
         url = service.source_url(source_kind, source_ref)
 
         _set_job(job_id, phase="downloading", progress=0)

frontend/src/components/DownloadCenter.tsx

@@ -36,8 +36,15 @@ const STATUS_CLS: Record<string, string> = {
   canceled: "text-muted",
 };
 
+const PHASE_KEYS = ["video", "audio", "merging", "processing"];
+
 function StatusPill({ job }: { job: DownloadJob }) {
   const { t } = useTranslation();
+  // While downloading, show the concrete phase (video / audio / merging) so the user isn't
+  // confused by the bar resetting between the separate video and audio streams.
+  if (job.status === "running" && job.phase && PHASE_KEYS.includes(job.phase)) {
+    return <span className="text-xs font-medium text-accent">{t(`downloads.phase.${job.phase}`)}</span>;
+  }
   const key = job.expired ? "expired" : job.status;
   return (
     <span className={clsx("text-xs font-medium", STATUS_CLS[job.status] ?? "text-muted")}>
@@ -109,6 +116,13 @@ function DownloadRow({
         </div>
         {running && (
           <div className="mt-1.5">
+            {job.phase === "merging" || job.phase === "processing" ? (
+              // No meaningful percentage during ffmpeg — show an indeterminate pulse.
+              <div className="h-1.5 rounded-full bg-surface overflow-hidden">
+                <div className="h-full w-full bg-accent/60 animate-pulse" />
+              </div>
+            ) : (
+              <>
                 <div className="h-1.5 rounded-full bg-surface overflow-hidden">
                   <div className="h-full bg-accent transition-all" style={{ width: `${job.progress}%` }} />
                 </div>
@@ -117,6 +131,8 @@ function DownloadRow({
                   {job.speed_bps ? <span>{formatSpeed(job.speed_bps)}</span> : null}
                   {job.eta_s ? <span>{formatEta(job.eta_s)}</span> : null}
                 </div>
+              </>
+            )}
           </div>
         )}
       </div>

frontend/src/i18n/locales/de/downloads.json

@@ -37,6 +37,12 @@
     "canceled": "Abgebrochen",
     "expired": "Abgelaufen"
   },
+  "phase": {
+    "video": "Video wird geladen",
+    "audio": "Audio wird geladen",
+    "merging": "Zusammenführen",
+    "processing": "Verarbeitung"
+  },
   "actions": {
     "pause": "Pause",
     "resume": "Fortsetzen",

frontend/src/i18n/locales/en/downloads.json

@@ -37,6 +37,12 @@
     "canceled": "Canceled",
     "expired": "Expired"
   },
+  "phase": {
+    "video": "Downloading video",
+    "audio": "Downloading audio",
+    "merging": "Merging",
+    "processing": "Processing"
+  },
   "actions": {
     "pause": "Pause",
     "resume": "Resume",

frontend/src/i18n/locales/hu/downloads.json

@@ -37,6 +37,12 @@
     "canceled": "Megszakítva",
     "expired": "Lejárt"
   },
+  "phase": {
+    "video": "Videósáv letöltése",
+    "audio": "Hangsáv letöltése",
+    "merging": "Összefűzés",
+    "processing": "Feldolgozás"
+  },
   "actions": {
     "pause": "Szünet",
     "resume": "Folytatás",