feat(premium): cryptographic session verification for premium bypass

[Xephi]

Requirements

• PacketEvents must be installed on the server, unless you
  are using proxy mode (see "Behind a proxy" on docs/premium.md).

How it works

When a premium-enrolled player connects, AuthMe intercepts the Minecraft login handshake at
the packet level:

Client                         Server (AuthMe + PacketEvents)        Mojang
  |                                  |                                 |
  |--LOGIN_START(name)-------------->|                                 |
  |                                  | ① DB: isPremium → true (async)  |
  |<--ENCRYPTION_REQUEST-------------|                                 |
  |  (RSA-1024 public key +          |                                 |
  |   random verify token)           |                                 |
  |  POST /session/minecraft/join ---------------------------------->  |
  |--ENCRYPTION_RESPONSE------------>|                                 |
  |  (enc(sharedSecret) +            |                                 |
  |   enc(verifyToken), RSA)         |                                 |
  |                                  | ② RSA-decrypt sharedSecret      |
  |                                  |   (sync, on event-loop)         |
  |                                  | ③ Install AES/CFB8 Netty ciphers|
  |                                  |   (sync — client already sends  |
  |                                  |    encrypted bytes from here)   |
  |                                  | ④ Verify token + GET /hasJoined |
  |                                  |   (async) --------------------->|
  |                                  |<--- {uuid, name, properties} ---|
  |                                  | ⑤ Store verified UUID (60 s TTL)|
  |                                  | ⑥ Re-inject LOGIN_START         |
  |     ... player joins PLAY ...    |                                 |
  |                                  |                                 |
  AsynchronousJoin: getVerifiedUuid(name) == auth.getPremiumUuid() → auto-login