feat: add ImageLoadingService for downloading series artwork

- Create ImageLoadingService that downloads poster.jpg, fanart.jpg, and
  logo.png from TMDB when anime is added or during scheduler rescan
- Integrate into BackgroundLoaderService._load_nfo_and_images() to trigger
  image downloads when new anime is added
- Add image_scan_after_rescan config option to scheduler (default: true)
- Add _run_image_scan() to scheduler rescan flow, processing series in
  batches of 10 to respect TMDB rate limits
- Fix SearchResult model missing folder, snippet, and score fields
- Update background_loader tests to match new image loading behavior

src/server/models/config.py

@@ -45,6 +45,11 @@ class SchedulerConfig(BaseModel):
                     "completes. Checks each series folder for tvshow.nfo and "
                     "creates or fills missing properties.",
     )
+    image_scan_after_rescan: bool = Field(
+        default=True,
+        description="Download series images (poster.jpg, fanart.jpg, logo.png) "
+                    "from TMDB after a scheduled rescan completes.",
+    )
     # Legacy alias fields — read via Pydantic alias
     auto_download: Optional[bool] = Field(default=None, alias="auto_download")