Signup Check

Pre-flight: is handle valid and available?

The signup page calls this as the user types. Same shape as /v1/handles/check but gated on the page password instead of a magic-link token. Does NOT claim the handle.

Body: {"password": "...", "handle": "leslie"}

Returns: {"available": true, "handle": "leslie", "full": "leslie@tandem-cc.com"} — or — {"available": false, "handle": "leslie", "suggestions": ["leslie2"]}