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.

tests/api/test_download_endpoints.py

@@ -146,7 +146,8 @@ async def test_get_queue_status_unauthorized(mock_download_service):
 async def test_add_to_queue(authenticated_client, mock_download_service):
     """Test POST /api/queue/add endpoint."""
     request_data = {
-        "serie_id": "series-1",
+        "serie_id": "test-anime",
+        "serie_folder": "Test Anime (2024)",
         "serie_name": "Test Anime",
         "episodes": [
             {"season": 1, "episode": 1},
@@ -175,7 +176,8 @@ async def test_add_to_queue_with_high_priority(
 ):
     """Test adding items with HIGH priority."""
     request_data = {
-        "serie_id": "series-1",
+        "serie_id": "test-anime",
+        "serie_folder": "Test Anime (2024)",
         "serie_name": "Test Anime",
         "episodes": [{"season": 1, "episode": 1}],
         "priority": "high",
@@ -198,7 +200,8 @@ async def test_add_to_queue_empty_episodes(
 ):
     """Test adding empty episodes list returns 400."""
     request_data = {
-        "serie_id": "series-1",
+        "serie_id": "test-anime",
+        "serie_folder": "Test Anime (2024)",
         "serie_name": "Test Anime",
         "episodes": [],
         "priority": "normal",
@@ -221,7 +224,8 @@ async def test_add_to_queue_service_error(
     )
 
     request_data = {
-        "serie_id": "series-1",
+        "serie_id": "test-anime",
+        "serie_folder": "Test Anime (2024)",
         "serie_name": "Test Anime",
         "episodes": [{"season": 1, "episode": 1}],
         "priority": "normal",