NIP-98 authorization parsing permits resource exhaustion

The NIP-98 HTTP authorization parser Base64-decoded the complete authorization value
and parsed the resulting JSON event without applying an application-level size limit.
Both operations occurred before the event's signature and authorization fields could
be validated.

An unauthenticated remote client could send an oversized `Authorization: Nostr`
header to a server using this parser. Each request caused memory allocation and
decoding and JSON parsing work proportional to the supplied value, allowing repeated
requests to consume server memory and CPU. The impact depends on any lower HTTP header
limit already enforced by the hosting server. The issue does not bypass NIP-98
authentication or disclose protected data.

The parser now rejects oversized encoded input before Base64 allocation and rejects
decoded authorization events larger than 64 KiB before JSON parsing.