feat(scripts): add historic-predictions deduplication tool

feat(scripts): add historic-predictions deduplication tool

Add scripts/historic_predictions_deduplicate.py to repair FreqAI
historic_predictions.pkl stores that a crash-recovery backfill can leave with
duplicate date_pred rows, which break the prediction merge for that pair.

- Keeps the most informative row per date_pred (informative cells, then non-null
  count, then most-recent write); deduplicates only non-NaT keys.
- Dry-run by default; --apply writes atomically (temp + fsync + os.replace,
  preserving mode and best-effort owner) after copying the original aside as
  .original-<stamp>. Exits 1 when any target is skipped as unreadable.
- Repairs the sibling historic_predictions.backup.pkl as well.
- Portable single-file tool with a bundled test suite run via uv.