Replace username/password authentication with FIDO2/WebAuthn passkey-based
auth using webauthn-rs. Sessions and bearer tokens are unchanged — only the
way they are created changes.
- Add webauthn-rs, uuid, open, url deps; remove argon2, rpassword
- Add passkey_credentials and registration_tokens tables (migrations 17-18)
- Add domain entities, typed IDs, and repository traits for passkeys/tokens
- Add SQL repository implementations for passkeys and registration tokens
- Add ChallengeStore for in-memory WebAuthn ceremony state
- Add WebAuthn route handlers (register/auth start+finish ceremonies)
- Add CLI browser handoff for token creation (opens browser, local callback)
- Replace login form with "Sign in with Passkey" button
- Add registration page for first-user bootstrap via one-time token
- Replace BREWLOG_ADMIN_USERNAME/PASSWORD with BREWLOG_RP_ID/RP_ORIGIN
- Change default BREWLOG_URL from 127.0.0.1 to localhost (WebAuthn requires it)
