fix(filter): use default=[] instead of default_factory for search_fields

FastAPI Depends() passes the factory sentinel as raw value instead of
calling it, causing ValidationError on LinkFilter query param injection.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ImTotem

src/bcsd_api/filter/base.py

@@ -11,7 +11,7 @@ class BaseFilter(BaseModel):
     sort_by: str = "id"
     sort_order: str = Field("asc", pattern="^(asc|desc)$")
 
-    search_fields: list[str] = Field(default_factory=list, exclude=True)
+    search_fields: list[str] = Field(default=[], exclude=True)
 
     def filters(self) -> dict:
         excluded = {"page", "size", "sort_by", "sort_order", "search_fields"}