Task 3.1: Standardize series identifiers in DownloadService

- Updated DownloadService to use 'serie_id' (provider key) for identification
- Changed 'serie_folder' from Optional to required in models (DownloadItem, DownloadRequest)
- Removed incorrect fallback logic that used serie_id as folder name
- Enhanced docstrings to clarify purpose of each identifier field:
  * serie_id: Provider key (e.g., 'attack-on-titan') for lookups
  * serie_folder: Filesystem folder name (e.g., 'Attack on Titan (2013)') for file operations
- Updated logging to reference 'serie_key' for clarity
- Fixed all unit tests to include required serie_folder field
- All 25 download service tests passing
- All 47 download model tests passing
- Updated infrastructure.md with detailed documentation
- Marked Task 3.1 as completed in instructions.md

Benefits:
- Clear separation between provider identifier and filesystem path
- Prevents confusion from mixing different identifier types
- Consistent with broader series identifier standardization effort
- Better error messages when required fields are missing

tests/unit/test_download_service.py

@@ -346,6 +346,7 @@ class TestQueueControl:
         completed_item = DownloadItem(
             id="completed-1",
             serie_id="series-1",
+            serie_folder="Test Series (2023)",
             serie_name="Test Series",
             episode=EpisodeIdentifier(season=1, episode=1),
             status=DownloadStatus.COMPLETED,
@@ -454,6 +455,7 @@ class TestRetryLogic:
         failed_item = DownloadItem(
             id="failed-1",
             serie_id="series-1",
+            serie_folder="Test Series (2023)",
             serie_name="Test Series",
             episode=EpisodeIdentifier(season=1, episode=1),
             status=DownloadStatus.FAILED,
@@ -476,6 +478,7 @@ class TestRetryLogic:
         failed_item = DownloadItem(
             id="failed-1",
             serie_id="series-1",
+            serie_folder="Test Series (2023)",
             serie_name="Test Series",
             episode=EpisodeIdentifier(season=1, episode=1),
             status=DownloadStatus.FAILED,