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"

src/server/services/download_service.py

@@ -239,11 +239,16 @@ class DownloadService:
         """Add episodes to the download queue (FIFO order).
 
         Args:
-            serie_id: Series identifier (provider key)
-            serie_folder: Series folder name on disk
-            serie_name: Series display name
+            serie_id: Series identifier - provider key (e.g.,
+                'attack-on-titan'). This is the unique identifier used
+                for lookups and identification.
+            serie_folder: Series folder name on disk (e.g.,
+                'Attack on Titan (2013)'). Used for filesystem operations
+                only.
+            serie_name: Series display name for user interface
             episodes: List of episodes to download
-            priority: Queue priority level (ignored, kept for compatibility)
+            priority: Queue priority level (ignored, kept for
+                compatibility)
 
         Returns:
             List of created download item IDs
@@ -277,7 +282,8 @@ class DownloadService:
                 logger.info(
                     "Item added to queue",
                     item_id=item.id,
-                    serie=serie_name,
+                    serie_key=serie_id,
+                    serie_name=serie_name,
                     season=episode.season,
                     episode=episode.episode,
                 )
@@ -792,7 +798,8 @@ class DownloadService:
             logger.info(
                 "Starting download",
                 item_id=item.id,
-                serie=item.serie_name,
+                serie_key=item.serie_id,
+                serie_name=item.serie_name,
                 season=item.episode.season,
                 episode=item.episode.episode,
             )
@@ -802,9 +809,15 @@ class DownloadService:
             # - download started/progress/completed/failed events
             # - All updates forwarded to ProgressService
             # - ProgressService broadcasts to WebSocket clients
-            folder = item.serie_folder if item.serie_folder else item.serie_id
+            # Use serie_folder for filesystem operations and serie_id (key) for identification
+            if not item.serie_folder:
+                raise DownloadServiceError(
+                    f"Missing serie_folder for download item {item.id}. "
+                    "serie_folder is required for filesystem operations."
+                )
+            
             success = await self._anime_service.download(
-                serie_folder=folder,
+                serie_folder=item.serie_folder,
                 season=item.episode.season,
                 episode=item.episode.episode,
                 key=item.serie_id,