Task 4.2: Update Download API Endpoints to Use Key

- Updated DownloadRequest and DownloadItem models with comprehensive
  docstrings explaining serie_id (key as primary identifier) vs
  serie_folder (filesystem metadata)
- Updated add_to_queue() endpoint docstring to document request parameters
- Updated all test files to include required serie_folder field:
  - tests/api/test_download_endpoints.py
  - tests/api/test_queue_features.py
  - tests/frontend/test_existing_ui_integration.py
  - tests/integration/test_download_flow.py
- Updated infrastructure.md with Download Queue request/response models
- All 869 tests pass

This is part of the Series Identifier Standardization effort (Phase 4.2)
to ensure key is used as the primary identifier throughout the codebase.

src/server/api/download.py

@@ -74,7 +74,12 @@ async def add_to_queue(
     Requires authentication.
 
     Args:
-        request: Download request with serie info, episodes, and priority
+        request: Download request containing:
+            - serie_id: Series key (primary identifier, 'attack-on-titan')
+            - serie_folder: Filesystem folder name for storing downloads
+            - serie_name: Display name for the series
+            - episodes: List of episodes to download
+            - priority: Queue priority level
 
     Returns:
         DownloadResponse: Status and list of created download item IDs