feat(store): persist passkey ceremony sessions in postgres

[jmgilman]

Summary

• add an authkit Postgres table and Store implementation for passkey ceremony sessions
• store WebAuthn SessionData as upstream MessagePack bytes inside the JSONB payload to preserve extension value types
• add shared session-store behavior coverage and wire testkit Postgres mode to use the auth Postgres store

Validation

• go test ./proof/passkey/session ./store/postgres
• go test -tags=integration ./store/postgres
• moon run root:check --summary minimal
• moon run root:integration --summary minimal
• git diff --check