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/anime.py

@@ -133,6 +133,29 @@ class SearchResult(BaseModel):
         )
     )
     title: str = Field(..., description="Series title")
+    folder: Optional[str] = Field(
+        None,
+        description=(
+            "Series folder name on disk (metadata only) "
+            "(e.g., 'Attack on Titan (2013)'). For display/filesystem ops only."
+        )
+    )
+    snippet: Optional[str] = Field(
+        None,
+        description="Search result snippet or description"
+    )
+    score: Optional[float] = Field(
+        None,
+        description="Search relevance score (0.0 to 1.0)"
+    )
+
+    @field_validator('key', mode='before')
+    @classmethod
+    def normalize_key(cls, v: str) -> str:
+        """Normalize key to lowercase."""
+        if isinstance(v, str):
+            return v.lower().strip()
+        return v
 
 
 class AnimeDetailsResponse(BaseModel):