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/downloads/formats.py

@@ -78,11 +78,16 @@ def normalize(spec: dict) -> dict:
     return s
 
 
-def format_sig(spec: dict) -> str:
-    """Stable short hash of the output-affecting spec — the MediaAsset cache key component."""
+def format_sig(spec: dict, salt: str | None = None) -> str:
+    """Stable short hash of the output-affecting spec — the MediaAsset cache key component.
+
+    `salt` forks a distinct, non-dedup identity for the same spec (used by force-redownload so it
+    always re-fetches into its own asset instead of sharing a cached file with other jobs)."""
     s = normalize(spec)
     data = {k: s[k] for k in _SIG_KEYS}
     data["_v"] = _SIG_VERSION
+    if salt is not None:
+        data["_salt"] = salt
     payload = json.dumps(data, sort_keys=True, separators=(",", ":"))
     return hashlib.sha256(payload.encode()).hexdigest()[:24]