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

src/server/models/download.py

@@ -66,9 +66,19 @@ class DownloadItem(BaseModel):
     """Represents a single download item in the queue."""
 
     id: str = Field(..., description="Unique download item identifier")
-    serie_id: str = Field(..., description="Series identifier (provider key)")
-    serie_folder: Optional[str] = Field(
-        None, description="Series folder name on disk"
+    serie_id: str = Field(
+        ...,
+        description=(
+            "Series identifier - provider key "
+            "(e.g., 'attack-on-titan')"
+        )
+    )
+    serie_folder: str = Field(
+        ...,
+        description=(
+            "Series folder name on disk "
+            "(e.g., 'Attack on Titan (2013)')"
+        )
     )
     serie_name: str = Field(..., min_length=1, description="Series name")
     episode: EpisodeIdentifier = Field(
@@ -160,9 +170,19 @@ class QueueStats(BaseModel):
 class DownloadRequest(BaseModel):
     """Request to add episode(s) to the download queue."""
 
-    serie_id: str = Field(..., description="Series identifier (provider key)")
-    serie_folder: Optional[str] = Field(
-        None, description="Series folder name on disk"
+    serie_id: str = Field(
+        ...,
+        description=(
+            "Series identifier - provider key "
+            "(e.g., 'attack-on-titan')"
+        )
+    )
+    serie_folder: str = Field(
+        ...,
+        description=(
+            "Series folder name on disk "
+            "(e.g., 'Attack on Titan (2013)')"
+        )
     )
     serie_name: str = Field(
         ..., min_length=1, description="Series name for display"