Fix download status flapping: only the 'completed' event is terminal

Production logs showed a job oscillating downloading -> completed ->
downloading -> completed -> postprocessing -> completed. Cause: yt-dlp
reports status='finished' via progress-hook ticks once per stream when
downloading separate video+audio for muxing (video lands, audio is still
in flight), and MeTube forwards that through 'updated' Socket.IO events
too -- we were mapping any status='finished' to our "completed", regardless
of which event carried it.

Only the dedicated 'completed' event (and the 'done' bucket of MeTube's
/history, for startup reconciliation) is now treated as authoritative for
terminal status; a transient 'finished' arriving via 'added'/'updated'
(or history's queue/pending) is a no-op for status, matching the
progress_percent update it also carries.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/app/services/download_jobs.py

@@ -106,18 +106,27 @@ async def handle_metube_event(db: Session, event_name: str, raw_payload) -> None
     if job is None:
         return
 
-    _apply_metube_info(job, payload)
+    # Only the dedicated 'completed' event is authoritative for terminal
+    # status. yt-dlp fires per-stream progress hooks with status=='finished'
+    # when downloading video+audio separately for merging (one stream lands,
+    # the other is still in flight) -- MeTube forwards that transient state
+    # through 'updated' events too, so treating it as our own "completed"
+    # there flaps the job back and forth: downloading -> completed ->
+    # downloading -> ... -> postprocessing -> completed. Verified against a
+    # real deploy's logs before this fix.
+    _apply_metube_info(job, payload, authoritative=event_name == "completed")
     db.commit()
     logger.info("download_job %s updated to status=%s via '%s' event", job.id, job.status, event_name)
 
 
-def _apply_metube_info(job: DownloadJob, info: dict) -> None:
+def _apply_metube_info(job: DownloadJob, info: dict, *, authoritative: bool) -> None:
     if info.get("id"):
         job.metube_job_id = info["id"]
 
-    our_status = METUBE_STATUS_MAP.get(info.get("status"), "unknown")
+    metube_status = info.get("status")
+    our_status = METUBE_STATUS_MAP.get(metube_status, "unknown")
 
-    if job.started_at is None and our_status in ("downloading", "postprocessing", "completed"):
+    if job.started_at is None and metube_status in ("downloading", "postprocessing"):
         job.started_at = datetime.now(timezone.utc)
 
     percent = info.get("percent")
@@ -128,6 +137,10 @@ def _apply_metube_info(job: DownloadJob, info: dict) -> None:
         job.status = "failed"
         job.error_message = info.get("msg") or info.get("error") or "Ошибка загрузки"
     elif our_status == "completed":
+        if not authoritative:
+            # A stream finished, not necessarily the whole job (see comment
+            # above) -- keep the current status, just take the progress tick.
+            return
         filename = info.get("filename")
         if filename:
             job.metube_filename = filename
@@ -135,7 +148,7 @@ def _apply_metube_info(job: DownloadJob, info: dict) -> None:
         job.status = "completed"
         job.progress_percent = 100
         job.completed_at = job.completed_at or datetime.now(timezone.utc)
-    else:
+    elif our_status != "unknown":
         job.status = our_status
 
 
@@ -158,22 +171,25 @@ def reconcile_on_startup(db: Session) -> None:
         db.commit()
         return
 
-    by_url: dict[str, dict] = {}
-    for bucket in ("queue", "pending", "done"):
+    # (item, authoritative) -- only the 'done' bucket is a terminal, trustworthy
+    # outcome; 'queue'/'pending' are still in flight (see handle_metube_event).
+    by_url: dict[str, tuple[dict, bool]] = {}
+    for bucket, authoritative in (("queue", False), ("pending", False), ("done", True)):
         for item in history.get(bucket, []) or []:
             url = item.get("url")
             if url:
-                by_url[url] = item
+                by_url[url] = (item, authoritative)
 
     videos = {v.id: v for v in db.query(Video).filter(Video.id.in_([j.video_id for j in jobs])).all()}
 
     for job in jobs:
         video = videos.get(job.video_id)
-        info = by_url.get(video.youtube_url) if video else None
-        if info is None:
+        match = by_url.get(video.youtube_url) if video else None
+        if match is None:
             job.status = "unknown"
             continue
-        _apply_metube_info(job, info)
+        info, authoritative = match
+        _apply_metube_info(job, info, authoritative=authoritative)
 
     db.commit()
     logger.info("Reconciled %d download job(s) against MeTube history", len(jobs))