[Waitlist] Username availability check

[Xaxxoo]

Overview

Allow users to check if a desired username is available before joining.

Tasks

• GET /api/v1/waitlist/check/:username (public)
• Return { available: boolean }
• Case-insensitive comparison
• Rate limit: 10 checks per minute per IP

Acceptance Criteria

• Returns available: false for taken usernames
• Case-insensitive (john = JOHN = John)
• Rate limited to prevent username enumeration