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_queue_features.py

@@ -48,6 +48,7 @@ def sample_download_request():
     """Sample download request for testing."""
     return {
         "serie_id": "test-series",
+        "serie_folder": "Test Series (2024)",
         "serie_name": "Test Series",
         "episodes": [
             {"season": 1, "episode": 1},
@@ -159,6 +160,7 @@ class TestQueueReordering:
                 "/api/queue/add",
                 json={
                     "serie_id": f"test-{i}",
+                    "serie_folder": f"Test Series {i} (2024)",
                     "serie_name": f"Test Series {i}",
                     "episodes": [{"season": 1, "episode": i+1}],
                     "priority": "normal"