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_naming.py

@@ -68,3 +68,11 @@ def test_engagement_only_title_does_not_resurrect_the_stats():
     out = normalize_title("1.6M views · 66K reactions |")
     assert out == ""
     assert "views" not in (out or "") and "reactions" not in (out or "")
+
+
+def test_prefix_plus_only_hashtags_resurrects_neither_stats_nor_hashtags():
+    # stats prefix + trailing SEO hashtags → both are removed; the fallback must not bring back the
+    # hashtags that rule 2 strips (nor the stats prefix).
+    out = normalize_title("5 views | #foo #bar")
+    assert out == ""
+    assert "#foo" not in (out or "") and "views" not in (out or "")