feat(downloads): granular post-processing phase labels

ffmpeg post-steps have no byte-progress, so instead of a silent 'Processing' the row now names
the current step (Merging / Extracting audio / Embedding thumbnail / Removing sponsors / Writing
metadata) with an indeterminate pulse. Byte-progress phases (video/audio) keep the % bar.
i18n en/hu/de.

backend/app/worker.py

@@ -246,15 +246,30 @@ def _make_progress_hook(job_id: int, asset_id: int):
     return hook
 
 
+def _pp_phase(pp: str) -> str:
+    """Map a yt-dlp postprocessor class name to a user-facing phase label. ffmpeg post-steps
+    aren't byte-progress operations (no %), so the UI shows the step name + an indeterminate
+    pulse — this makes the long merge/finalize legible instead of a silent 'Processing'."""
+    if "Merg" in pp:
+        return "merging"
+    if "ExtractAudio" in pp:
+        return "audio_extract"
+    if "Thumbnail" in pp:
+        return "thumbnail"
+    if "SponsorBlock" in pp or "ModifyChapters" in pp:
+        return "sponsorblock"
+    if "Metadata" in pp:
+        return "metadata"
+    return "processing"
+
+
 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."""
+    """Postprocessor hook: surface the current ffmpeg step so the bar shows a concrete label
+    (Merging / Embedding thumbnail / …) 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)
+            _set_job(job_id, phase=_pp_phase(d.get("postprocessor") or ""), speed_bps=None, eta_s=None)
 
     return hook