Reserve Handle

One-shot: turns a pending signup into a live Tandem user.

Wraps three steps in a single transaction so an error anywhere rolls everything back:

1. UPDATE the reservation row (used_at, handle_issued). A
   concurrent request that beat us loses the race — 0 rows
   affected = 410 Gone.
2. INSERT into handles. Unique violation = 409 Conflict (handle
   was just claimed in a different /reserve call).
3. INSERT into tokens bound to the new handle.

Returns { handle, token, email } with the raw bearer token. Token is never shown again.