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/TESTING.md

@@ -62,6 +62,52 @@ This document describes the testing strategy, guidelines, and practices for the
     - What to mock
     - Mock patterns
     - External service mocks
+
+### Mocking the Download Queue
+
+Use `MockQueueRepository` for testing download queue functionality:
+
+```python
+from src.server.models.download import DownloadItem, EpisodeIdentifier
+
+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_item(self, item_id: str) -> Optional[DownloadItem]:
+        return self._items.get(item_id)
+
+    async def get_all_items(self) -> List[DownloadItem]:
+        return list(self._items.values())
+
+    async def set_error(self, item_id: str, error: str) -> bool:
+        if item_id in self._items:
+            self._items[item_id].error = error
+            return True
+        return False
+
+    async def delete_item(self, item_id: str) -> bool:
+        if item_id in self._items:
+            del self._items[item_id]
+            return True
+        return False
+
+    async def clear_all(self) -> int:
+        count = len(self._items)
+        self._items.clear()
+        return count
+```
+
+**Key points:**
+- The mock uses in-memory storage, no database required
+- All async methods are implemented (even if just pass-through)
+- `save_item` uses `item.id` as key (must be set before calling)
+- Suitable for unit tests only (no persistence)
+
 7. Coverage Requirements
 8. CI/CD Integration
 9. Writing Good Tests