fix: enable official bots to connect to external tournament server (#71)

Two bugs prevented official bots from joining the external tournament-server:

1. JWT claim mismatch — bot tokens lacked the `isBot: true` claim the
   tournament server requires. Added the claim to generateBotToken() in
   AccountService, which covers both user-owned bots and official bots.

2. Broken join flow — TournamentBotGamePlayer.joinTournament() called
   registerBot() which hit POST /api/auth/register on the tournament server,
   an endpoint that does not exist. Removed registerBot() and updated
   JoinTournamentRequest to accept a botToken field so the caller supplies
   the pre-existing NowChessSystems token directly.

---------

Co-authored-by: LQ63 <lkhermann@web.de>
Reviewed-on: #71
Co-authored-by: Leon Hermann <lq@blackhole.local>
Co-committed-by: Leon Hermann <lq@blackhole.local>

modules/account/src/main/scala/de/nowchess/account/service/AccountService.scala

@@ -239,6 +239,7 @@ class AccountService:
       .subject(botId.toString)
       .expiresAt(Long.MaxValue)
       .claim("type", "bot")
+      .claim("isBot", true)
       .claim("name", botName)
       .sign()