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/titles.py

@@ -58,8 +58,9 @@ def normalize_title(raw: str | None) -> str | None:
         return raw
     t = unicodedata.normalize("NFKC", raw)
     t = "".join(ch for ch in t if unicodedata.category(ch) not in _DROP_CATEGORIES)
-    t = _ENGAGEMENT_PREFIX.sub("", t)
-    t = _TRAILING_HASHTAGS.sub("", t)
+    stripped = _ENGAGEMENT_PREFIX.sub("", t)
+    prefix_removed = stripped != t
+    t = _TRAILING_HASHTAGS.sub("", stripped)
 
     multi = [r for r in _LETTER_RUN.findall(t) if len(r) >= 2]
     caps = [r for r in multi if r.isupper()]
@@ -87,4 +88,7 @@ def normalize_title(raw: str | None) -> str | None:
             if ch.isalpha():
                 t = t[:i] + ch.upper() + t[i + 1:]
                 break
-    return t or raw
+    # Nothing survived cleaning: fall back to the engagement-stripped text (never resurrect a
+    # stats prefix). Only use the untouched raw when no prefix was there to strip — e.g. an
+    # all-emoji title whose glyphs were dropped, where returning the original is the lesser evil.
+    return t or stripped.strip() or ("" if prefix_removed else raw)