Refactor: Defer folder creation to download time

- Remove folder creation from add_series endpoint
- Add folder creation to download() method in SeriesApp
- Maintain database persistence and targeted scanning
- Update tests to use tmp_path fixtures
- All add_series and download tests passing (13/13)

src/server/api/anime.py

@@ -739,8 +739,7 @@ async def add_series(
                 db_id
             )
         
-        # Step D: Create folder on disk and add to SerieList
-        folder_path = None
+        # Step D: Add to SerieList (in-memory only, no folder creation)
         if series_app and hasattr(series_app, "list"):
             serie = Serie(
                 key=key,
@@ -750,25 +749,15 @@ async def add_series(
                 episodeDict={}
             )
             
-            # Add to SerieList - this creates the folder with sanitized name
-            if hasattr(series_app.list, 'add'):
-                with warnings.catch_warnings():
-                    warnings.simplefilter("ignore", DeprecationWarning)
-                    folder_path = series_app.list.add(serie, use_sanitized_folder=True)
-                    # Update folder to reflect what was actually created
-                    folder = serie.folder
-            elif hasattr(series_app.list, 'keyDict'):
-                # Manual folder creation and cache update
-                if hasattr(series_app.list, 'directory'):
-                    folder_path = os.path.join(series_app.list.directory, folder)
-                    os.makedirs(folder_path, exist_ok=True)
+            # Add to in-memory cache without creating folder on disk
+            if hasattr(series_app.list, 'keyDict'):
                 series_app.list.keyDict[key] = serie
-            
-            logger.info(
-                "Created folder for series: %s at %s",
-                name,
-                folder_path or folder
-            )
+                logger.info(
+                    "Added series to in-memory cache: %s (key=%s, folder=%s)",
+                    name,
+                    key,
+                    folder
+                )
         
         # Step E: Trigger targeted scan for missing episodes
         try:
@@ -818,7 +807,7 @@ async def add_series(
             "status": "success",
             "message": f"Successfully added series: {name}",
             "key": key,
-            "folder": folder_path or folder,
+            "folder": folder,
             "db_id": db_id,
             "missing_episodes": missing_episodes_serializable,
             "total_missing": total_missing