fix(downloads): address code-review findings on the R2.5 mini-epic

- redownload: never clobber a co-held (deduped) asset. Drop the force-reset-to-pending;
  _release_asset already deletes+recreates a solo asset (genuine re-fetch), while a shared
  ready asset is left intact and the re-queued job re-points to it via _finish_from_cache —
  so one user's re-download can't delete another's file mid-serve (finding #1).
- redownload: reset queue_pos to the tail so a re-download doesn't front-run newer queued
  jobs on the worker's `ORDER BY queue_pos` claim (finding #3). Made service.next_queue_pos public.
- formats: video-only mp4 selector prefers every video-only option before any progressive
  `best`, so a "Video only" download stays audio-free whenever a video-only stream exists (#2).
- titles: a title that is only the engagement prefix no longer resurrects the stripped stats
  (return the stripped text / empty, not the raw) (#4).
- worker: probe the file's duration when yt-dlp gave none, so a long re-encode shows real
  progress instead of a frozen 0% (#5); tighten the ensure_browser_playable docstring to its
  actual mp4-profile scope (#6).

backend/tests/test_formats.py

@@ -26,6 +26,9 @@ def test_video_only_mp4_selects_avc1():
     fmt = _opts({"mode": "v", "container": "mp4"})["format"]
     assert fmt.startswith("bestvideo[vcodec^=avc1]")
     assert "+bestaudio" not in fmt  # video-only never merges audio
+    # A video-only download stays audio-free: every `bestvideo` (video-only) option is preferred
+    # over any `best` (progressive, audio-bearing) fallback.
+    assert fmt.index("bestvideo") < fmt.index("best[vcodec^=avc1]") < fmt.rindex("/best")
 
 
 def test_max_height_caps_every_branch():

backend/tests/test_naming.py

@@ -61,3 +61,10 @@ def test_engagement_prefix_does_not_touch_normal_titles():
     # A pipe that isn't an engagement prefix must survive.
     assert normalize_title("Vlog 12 | A nagy nap") == "Vlog 12 | A nagy nap"
     assert normalize_title("How I built a boat") == "How I built a boat"
+
+
+def test_engagement_only_title_does_not_resurrect_the_stats():
+    # A title that is ONLY the stats prefix must not fall back to the raw (stats-bearing) string.
+    out = normalize_title("1.6M views · 66K reactions |")
+    assert out == ""
+    assert "views" not in (out or "") and "reactions" not in (out or "")