feat(misc): support seasonal backgrounds

app/config.py

@@ -55,6 +55,7 @@ class Settings(BaseSettings):
     enable_all_mods_pp: bool = False
     enable_supporter_for_all_users: bool = False
     enable_all_beatmap_leaderboard: bool = False
+    seasonal_backgrounds: list[str] = []
 
     @field_validator("fetcher_scopes", mode="before")
     def validate_fetcher_scopes(cls, v: Any) -> list[str]: