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/service.py

@@ -46,6 +46,14 @@ def get_or_create_asset(
     )
     asset = db.execute(stmt).scalar_one_or_none()
     if asset is not None:
+        # A previously-FAILED shared asset must be re-attempted for the new job about to hold it —
+        # reset it to pending (+clear the error) so the worker actually re-downloads instead of
+        # short-circuiting the new job with the asset's stale error. (Mirrors resume_download; without
+        # this a fresh enqueue of a once-failed (source,format) pair is permanently poisoned, since
+        # errored assets carry no expires_at and GC never clears them.)
+        if asset.status == "error":
+            asset.status = "pending"
+            asset.error = None
         return asset
     asset = MediaAsset(
         source_kind=source_kind,