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/app/downloads/formats.py

@@ -152,9 +152,12 @@ def build_ydl_opts(
         compat = s["container"] == "mp4" and s["vcodec"] is None
         if s["mode"] == "v":
             if compat:
+                # Video-only: keep it audio-free. Prefer an H.264 *video-only* stream, then any
+                # video-only, and only fall to a progressive (audio-bearing) `best` as a last
+                # resort — same audio-free-when-possible guarantee as the old `bestvideo/best`.
                 opts["format"] = (
-                    f"bestvideo[vcodec^=avc1]{hcap}/best[vcodec^=avc1]{hcap}"
-                    f"/bestvideo{hcap}/best{hcap}"
+                    f"bestvideo[vcodec^=avc1]{hcap}/bestvideo{hcap}"
+                    f"/best[vcodec^=avc1]{hcap}/best{hcap}"
                 )
             else:
                 opts["format"] = f"bestvideo{hcap}/best{hcap}"