The MyAnimeList API inconsistently returns statistics status fields (watching,
completed, on_hold, dropped, plan_to_watch) as quoted strings for non-zero
values (e.g. "8217") but as bare numbers for zero values (e.g. 0). This caused
JSON unmarshal errors for anime with zero counts in any status field.
Introduce a FlexString custom type that implements json.Unmarshaler to accept
both JSON strings and JSON numbers, always storing the result as a string. The
type definition lives in MALTypes.go and the unmarshal logic in MALFunctions.go
to keep static types and behavior separate.
