fix(downloads): repair 5 confirmed backend bugs (B1-B5)

- B1 (sticky errored asset): get_or_create_asset now resets a reused status=='error'
  asset back to 'pending' (+clears the error), so a fresh enqueue/edit of a once-failed
  (source,format) pair actually re-downloads instead of the worker short-circuiting the
  new job with the stale error. Errored assets carry no expires_at, so without this the
  pair was permanently poisoned for all users until someone hit resume.
- B2 (ref_count leak): _release_asset counts 'error' as a holding state, so deleting an
  errored job decrements the ref_count that enqueue always incremented (the worker never
  decrements on failure). Errored rows are deliberately NOT deleted here — a concurrent
  B1 reuse could otherwise be lost-updated + FK-nulled; the row is fileless and harmless.
- B3: admin storage dashboard reads sysconfig.get_int(db,'download_total_max_bytes')
  (the admin-editable DB value GC enforces) instead of the raw env default.
- B4: the single-trim branch of normalize_edit_spec guards its float() coercion like the
  crop/segments branches — a malformed trim now yields a 400, not an unhandled 500.
- B5: formats.normalize guards int(max_height) → falls back to "best" instead of 500.

Reviewed (race in an earlier B2 draft caught + fixed); localdev boots, B4/B5 unit-verified.

backend/app/downloads/formats.py

@@ -62,7 +62,12 @@ def normalize(spec: dict) -> dict:
     if s["mode"] not in ("av", "v", "a"):
         s["mode"] = "av"
     if s["max_height"] is not None:
-        s["max_height"] = int(s["max_height"])
+        # A malformed max_height (non-numeric inline spec / stored profile) falls back to "best"
+        # rather than raising an unhandled 500 at enqueue.
+        try:
+            s["max_height"] = int(s["max_height"])
+        except (TypeError, ValueError):
+            s["max_height"] = None
     for b in ("embed_subs", "embed_chapters", "embed_thumbnail", "sponsorblock"):
         s[b] = bool(s[b])
     # Audio-only files can't embed subtitles/thumbnails as video; keep the flags meaningful.