feat(download): persist retry state and dead-letter

Retry count and queue status were in-memory only and lost on
restart, so failed downloads could not be safely resumed and
permanently-failed episodes silently blocked re-queueing via the
episode-id unique index.

- Add status + retry_count columns to DownloadQueueItem
- Replace unique(episode_id) with unique(episode_id, status) so
  permanently_failed rows do not block new pending entries
- Add PERMANENTLY_FAILED to DownloadStatus enum
- Persist retry_count on each failure; mark permanently_failed once
  max_retries reached
- QueueRepository reads status/retry_count from DB instead of
  defaulting to PENDING/0
- Stop double-incrementing retry_count in retry_failed_items;
  increment only happens in _process_download on failure

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/unit/test_queue_repository.py

@@ -70,6 +70,8 @@ def _make_db_item(
     completed_at: datetime | None = None,
     error_message: str | None = None,
     download_url: str | None = None,
+    status: str = "pending",
+    retry_count: int = 0,
 ):
     """Build a fake DB DownloadQueueItem."""
     episode = MagicMock()
@@ -91,6 +93,8 @@ def _make_db_item(
     db_item.completed_at = completed_at
     db_item.error_message = error_message
     db_item.download_url = download_url
+    db_item.status = status
+    db_item.retry_count = retry_count
     return db_item