fix(downloads): second-round review fixes (fork on re-download, title fallbacks)

- redownload now ALWAYS rebuilds into a fresh private asset instead of ever reusing a shared
  cached file: the cache sig is salted with job_id + current asset_id, so the fork is distinct
  from both the dedup pool and the job's own current asset. This resolves the co-held silent
  no-op (finding #1) — a shared asset is left intact for its other holders while THIS job
  re-fetches its own copy. Resolving the fork BEFORE releasing the old hold (and the unique
  salted key) also removes the IntegrityError-rollback-after-file-delete window (finding #2).
- format_sig gains an optional `salt` for that fork identity.
- titles: don't consume the space after `|`, so a "stats | #hashtags" remainder keeps the
  leading whitespace _TRAILING_HASHTAGS needs — and fall back to the prefix+hashtag-stripped
  text, so neither the stats nor the SEO hashtags are resurrected when a title empties (#3).
- worker: early-fill asset.title falls back to the video id so a title that cleaned to empty
  doesn't show a blank card while downloading (#5).
- Tests for the salt fork and the hashtag-remainder case.

(Finding #4 — quota on re-download — intentionally not changed: a re-download reuses the
existing job and re-fetches ~the same bytes, so calling check_enqueue would wrongly reject on
the job-count cap while footprint is unchanged.)

backend/tests/test_formats.py

@@ -17,6 +17,16 @@ def test_mp4_default_selects_avc1_with_progressive_fallback():
     assert fmt.endswith("bestvideo+bestaudio/best")
 
 
+def test_format_sig_salt_forks_a_distinct_identity():
+    spec = {"mode": "av", "container": "mp4", "max_height": 720}
+    base = formats.format_sig(spec)
+    # A salt yields a distinct cache identity (force-redownload's private-asset fork); different
+    # salts differ from each other and from the unsalted sig, same salt is stable.
+    assert formats.format_sig(spec, salt="7:12") != base
+    assert formats.format_sig(spec, salt="7:12") != formats.format_sig(spec, salt="7:13")
+    assert formats.format_sig(spec, salt="7:12") == formats.format_sig(spec, salt="7:12")
+
+
 def test_mp4_default_keeps_h264_sort_tiebreaker():
     sort = _opts({"mode": "av", "container": "mp4"})["format_sort"]
     assert "vcodec:h264" in sort and "acodec:aac" in sort