Places the fixed-cost bet into the current round: builds and broadcasts
the user->pool PSBT with change back to the user's own address, enforces
at most one active bet per user, and registers the confirmation handler
that marks a bet confirmed and adds the participant to the round.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Argon2 password hashing, JWT session issuing/verification
(auth/security.py), register/login routes, the bearer-token
get_current_user dependency, and GET /users/me for address + balance.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>