Add queue deduplication to prevent duplicate entries

- In-memory dedup in add_to_queue() using _pending_by_episode dict
- Batch-local dedup via seen_in_batch set (handles duplicates within single call)
- Database unique index on episode_id via __table_args__
- 5-minute cooldown in _auto_download_missing() to prevent rapid re-triggers
- Updated _add_to_pending_queue() and _remove_from_pending_queue() to track episode keys
- Added TestQueueDeduplication with 4 test cases
- Updated DEVELOPMENT.md and TESTING.md with queue dedup docs

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

docs/DEVELOPMENT.md

@@ -61,4 +61,56 @@ This document provides guidance for developers working on the Aniworld project.
     - Commit message format
     - Pull request process
 8. Common Development Tasks
+
+### Adding Queue Deduplication
+
+The download queue prevents duplicate entries at two levels:
+
+**In-Memory Deduplication** (`src/server/services/download_service.py`):
+- `_pending_by_episode` dict tracks pending episodes: key = `(serie_id, season, episode)`
+- `_add_to_pending_queue()` updates the dict when adding items
+- `add_to_queue()` checks this dict before adding episodes (includes batch-local dedup)
+- `_remove_from_pending_queue()` cleans up the dict when items are removed
+
+**Database Constraint** (`src/server/models.py`):
+- `DownloadQueueItem` has a unique index on `episode_id` via `__table_args__`
+- Prevents duplicate queue entries at the database level
+- Unique constraint: `Index("ix_download_queue_episode_pending", "episode_id", unique=True)`
+
+**Scheduler Cooldown** (`src/server/services/scheduler_service.py`):
+- `_last_auto_download_time` tracks when auto-download last ran
+- 5-minute cooldown prevents rapid re-triggers
+- Checked at start of `_auto_download_missing()`
+
+### Mocking the Download Queue
+
+When testing components that use the download queue:
+
+```python
+# Mock repository for unit tests
+class MockQueueRepository:
+    def __init__(self):
+        self._items: Dict[str, DownloadItem] = {}
+
+    async def save_item(self, item: DownloadItem) -> DownloadItem:
+        self._items[item.id] = item
+        return item
+
+    async def get_all_items(self) -> List[DownloadItem]:
+        return list(self._items.values())
+
+# Use in fixture
+@pytest.fixture
+def mock_queue_repository():
+    return MockQueueRepository()
+
+@pytest.fixture
+def download_service(mock_anime_service, mock_queue_repository):
+    return DownloadService(
+        anime_service=mock_anime_service,
+        queue_repository=mock_queue_repository,
+        max_retries=3,
+    )
+```
+
 9. Troubleshooting Development Issues