OpenReception's bootstrap challenge proof-of-work difficulty hardcoded to 16 bits, which enables abuse rate amplification

OpenReception's appointment booking software provides an end-to-end encrypted appointment booking platform. Prior to version 1.0.6, the bootstrap challenge endpoint at `/api/tenants/{id}/appointments/bootstrap-challenge` issues a SHA-256 proof-of-work with `difficulty=4` hex zeros, equivalent to 16 bits of work. Modern hardware solves this in under 200 milliseconds, providing essentially no friction against automated abuse of the patient booking flow. Proof-of-work is used in the booking flow as a rate-limiter for unauthenticated clients establishing tunnels and submitting appointments. At 16 bits of difficulty, the construct is decorative rather than effective. An attacker can solve PoW challenges as fast as the server can issue them, defeating the rate-limiting purpose. The handler also calls `challengeThrottleService.checkThrottle(binding, "passkey")`, but the binding includes attacker-controlled values (`tunnelId`, `clientPublicKey`, and optional `emailHash`). For each fresh attempt, the attacker can supply new values, producing a new throttle key and bypassing the per-binding accumulation. Practical abuse friction is therefore the PoW difficulty itself, not a stable per-IP or per-email server-side throttle. Version 1.0.6 fixes the issue.