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/edit.py

@@ -79,13 +79,17 @@ def normalize_edit_spec(spec: dict | None) -> dict:
     # Single TRIM (one output file; the frontend fans a "separate" split out into N of these).
     trim = spec.get("trim") or {}
     if trim:
-        start = max(0.0, float(trim.get("start_s") or 0.0))
-        end_raw = trim.get("end_s")
+        # Guard the numeric coercion like the crop/segments branches do — a malformed value must
+        # drop the trim (→ empty spec → 400 EditError), not raise an unhandled 500.
+        try:
+            start = max(0.0, float(trim.get("start_s") or 0.0))
+            end_raw = trim.get("end_s")
+            end = float(end_raw) if end_raw is not None else None
+        except (TypeError, ValueError):
+            start, end = 0.0, None
         t: dict = {"start_s": round(start, 3)}
-        if end_raw is not None:
-            end = float(end_raw)
-            if end > start:
-                t["end_s"] = round(end, 3)
+        if end is not None and end > start:
+            t["end_s"] = round(end, 3)
         # A trim with only a start (open-ended) is valid (cut to the end).
         if t.get("start_s") or "end_s" in t:
             out["trim"] = t