[Waitlist] Waitlist position and signup stats

[Xaxxoo]

Overview

Show users their queue position and display total waitlist signups publicly.

Tasks

• GET /api/v1/waitlist/stats — total count (public)
• GET /api/v1/waitlist/position?email=xxx — position number
• Position based on createdAt chronological order
• Cache stats for 60 seconds

Acceptance Criteria

• Position is accurate and 1-indexed
• Unknown email returns 404
• Stats cached, not recomputed on every request