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/app/worker.py

@@ -249,7 +249,10 @@ def _fill_asset_meta_early(asset_id: int, info: dict) -> None:
         asset = db.get(MediaAsset, asset_id)
         if asset is None or asset.title:
             return
-        asset.title = normalize_title(info.get("title"))
+        # `or info.get("id")` avoids a blank card while downloading when the cleaned title is empty
+        # (e.g. a title that was only a social engagement prefix); the completion path later sets
+        # the final title with its own source_ref fallback.
+        asset.title = normalize_title(info.get("title")) or info.get("id")
         asset.uploader = info.get("uploader") or info.get("channel")
         asset.uploader_url = _uploader_url(info)
         asset.thumbnail_url = info.get("thumbnail")