Task 4.5: Update Pydantic models to use key as primary identifier

- Updated AnimeSeriesResponse and SearchResult models in anime.py:
  - Changed 'id' field to 'key' as the primary series identifier
  - Added 'folder' as optional metadata field
  - Added field validator to normalize key to lowercase and strip whitespace
  - Added comprehensive docstrings explaining identifier usage

- Updated DownloadItem and DownloadRequest models in download.py:
  - Added field validator for serie_id normalization (lowercase, stripped)
  - Improved documentation for serie_id (primary identifier) vs serie_folder (metadata)

- Updated test_anime_models.py with comprehensive tests:
  - Tests for key normalization and whitespace stripping
  - Tests for folder as optional metadata
  - Reorganized tests into proper class structure

- Updated test_download_models.py with validator tests:
  - Tests for serie_id normalization in DownloadItem
  - Tests for serie_id normalization in DownloadRequest

All 885 tests pass.

src/server/models/download.py

@@ -126,6 +126,14 @@ class DownloadItem(BaseModel):
         None, description="Source URL for download"
     )
 
+    @field_validator('serie_id', mode='before')
+    @classmethod
+    def normalize_serie_id(cls, v: str) -> str:
+        """Normalize serie_id (key) to lowercase and stripped."""
+        if isinstance(v, str):
+            return v.lower().strip()
+        return v
+
 
 class QueueStatus(BaseModel):
     """Overall status of the download queue system."""
@@ -218,6 +226,14 @@ class DownloadRequest(BaseModel):
             return v.upper()
         return v
 
+    @field_validator('serie_id', mode='before')
+    @classmethod
+    def normalize_serie_id(cls, v: str) -> str:
+        """Normalize serie_id (key) to lowercase and stripped."""
+        if isinstance(v, str):
+            return v.lower().strip()
+        return v
+
 
 class DownloadResponse(BaseModel):
     """Response after adding items to the download queue."""