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/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)