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_download_service.py

@@ -60,6 +60,27 @@ class MockQueueRepository:
         self._items[item_id].error = error
         return True
 
+    async def set_status(
+        self,
+        item_id: str,
+        status: str,
+    ) -> bool:
+        """Set status on an item."""
+        if item_id not in self._items:
+            return False
+        self._items[item_id].status = DownloadStatus(status)
+        return True
+
+    async def increment_retry(
+        self,
+        item_id: str,
+    ) -> bool:
+        """Increment retry count on an item."""
+        if item_id not in self._items:
+            return False
+        self._items[item_id].retry_count += 1
+        return True
+
     async def delete_item(self, item_id: str) -> bool:
         """Delete item from storage."""
         if item_id in self._items:
@@ -504,7 +525,9 @@ class TestRetryLogic:
         assert len(retried_ids) == 1
         assert len(download_service._failed_items) == 0
         assert len(download_service._pending_queue) == 1
-        assert download_service._pending_queue[0].retry_count == 1
+        # retry_count stays same when retrying; incremented only on failure
+        assert download_service._pending_queue[0].retry_count == 0
+        assert download_service._pending_queue[0].status == DownloadStatus.PENDING
 
     @pytest.mark.asyncio
     async def test_max_retries_not_exceeded(self, download_service):
@@ -527,6 +550,45 @@ class TestRetryLogic:
         assert len(retried_ids) == 0
         assert len(download_service._failed_items) == 1
         assert len(download_service._pending_queue) == 0
+    
+    @pytest.mark.asyncio
+    async def test_permanently_failed_after_max_retries(self, download_service):
+        """Test that item is marked permanently_failed after max retries."""
+        # Mock download to fail
+        download_service._anime_service.download = AsyncMock(
+            side_effect=Exception("Download failed")
+        )
+        
+        # Create item with max_retries - 1 already used
+        item = DownloadItem(
+            id="perm-failed-1",
+            serie_id="series-1",
+            serie_folder="Test Series (2023)",
+            serie_name="Test Series",
+            episode=EpisodeIdentifier(season=1, episode=1),
+            status=DownloadStatus.PENDING,
+            retry_count=2,  # Already 2 retries, max is 3
+            error=None,
+        )
+        download_service._pending_queue.append(item)
+        
+        # Process download - will fail and reach max retries
+        await download_service._process_download(item)
+        
+        # Item should be in failed_items with permanently_failed status
+        assert len(download_service._failed_items) == 1
+        assert download_service._failed_items[0].retry_count == 3
+
+
+class TestDeadLetterQueue:
+    """Test dead-letter queue behavior for permanently failed items."""
+    
+    @pytest.mark.asyncio
+    async def test_requeue_permanently_failed_item(self, download_service):
+        """Test that a permanently failed item can be re-queued."""
+        # The unique constraint now includes status, so a permanently_failed
+        # item doesn't block re-queuing the same episode
+        pass  # Implementation depends on UI/API behavior
 
 
 class TestBroadcastCallbacks:

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