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_models.py

@@ -174,6 +174,7 @@ class TestDownloadItem:
         item = DownloadItem(
             id="download_123",
             serie_id="serie_456",
+            serie_folder="Test Series (2023)",
             serie_name="Test Series",
             episode=episode,
             status=DownloadStatus.PENDING,
@@ -192,6 +193,7 @@ class TestDownloadItem:
         item = DownloadItem(
             id="test_id",
             serie_id="serie_id",
+            serie_folder="Test Folder",
             serie_name="Test",
             episode=episode
         )
@@ -211,6 +213,7 @@ class TestDownloadItem:
         item = DownloadItem(
             id="test_id",
             serie_id="serie_id",
+            serie_folder="Test Folder",
             serie_name="Test",
             episode=episode,
             progress=progress
@@ -225,6 +228,7 @@ class TestDownloadItem:
         item = DownloadItem(
             id="test_id",
             serie_id="serie_id",
+            serie_folder="Test Folder",
             serie_name="Test",
             episode=episode,
             started_at=now,
@@ -240,6 +244,7 @@ class TestDownloadItem:
             DownloadItem(
                 id="test_id",
                 serie_id="serie_id",
+                serie_folder="Test Folder",
                 serie_name="",
                 episode=episode
             )
@@ -251,6 +256,7 @@ class TestDownloadItem:
             DownloadItem(
                 id="test_id",
                 serie_id="serie_id",
+                serie_folder="Test Folder",
                 serie_name="Test",
                 episode=episode,
                 retry_count=-1
@@ -263,6 +269,7 @@ class TestDownloadItem:
         item = DownloadItem(
             id="test_id",
             serie_id="serie_id",
+            serie_folder="Test Folder",
             serie_name="Test",
             episode=episode
         )
@@ -279,6 +286,7 @@ class TestQueueStatus:
         item = DownloadItem(
             id="test_id",
             serie_id="serie_id",
+            serie_folder="Test Folder",
             serie_name="Test",
             episode=episode
         )
@@ -375,6 +383,7 @@ class TestDownloadRequest:
         episode2 = EpisodeIdentifier(season=1, episode=2)
         request = DownloadRequest(
             serie_id="serie_123",
+            serie_folder="Test Series (2023)",
             serie_name="Test Series",
             episodes=[episode1, episode2],
             priority=DownloadPriority.HIGH
@@ -389,16 +398,21 @@ class TestDownloadRequest:
         episode = EpisodeIdentifier(season=1, episode=1)
         request = DownloadRequest(
             serie_id="serie_123",
+            serie_folder="Test Series (2023)",
             serie_name="Test Series",
             episodes=[episode]
         )
         assert request.priority == DownloadPriority.NORMAL
 
     def test_empty_episodes_list_allowed(self):
-        """Test that empty episodes list is allowed at model level (endpoint validates)."""
+        """Test that empty episodes list is allowed at model level.
+        
+        (endpoint validates)
+        """
         # Empty list is now allowed at model level; endpoint validates
         request = DownloadRequest(
             serie_id="serie_123",
+            serie_folder="Test Series (2023)",
             serie_name="Test Series",
             episodes=[]
         )
@@ -410,6 +424,7 @@ class TestDownloadRequest:
         with pytest.raises(ValidationError):
             DownloadRequest(
                 serie_id="serie_123",
+                serie_folder="Test Series (2023)",
                 serie_name="",
                 episodes=[episode]
             )
@@ -453,7 +468,10 @@ class TestQueueOperationRequest:
         assert "item1" in request.item_ids
 
     def test_empty_item_ids_allowed(self):
-        """Test that empty item_ids list is allowed at model level (endpoint validates)."""
+        """Test that empty item_ids list is allowed at model level.
+        
+        (endpoint validates)
+        """
         # Empty list is now allowed at model level; endpoint validates
         request = QueueOperationRequest(item_ids=[])
         assert request.item_ids == []
@@ -512,6 +530,7 @@ class TestModelSerialization:
         item = DownloadItem(
             id="test_id",
             serie_id="serie_id",
+            serie_folder="Test Series (2023)",
             serie_name="Test Series",
             episode=episode
         )
@@ -526,6 +545,7 @@ class TestModelSerialization:
         data = {
             "id": "test_id",
             "serie_id": "serie_id",
+            "serie_folder": "Test Series (2023)",
             "serie_name": "Test Series",
             "episode": {
                 "season": 1,