Signup no longer fails on default-project domain collisions

Termerly creates a default project for every new user at signup, with a generated domain placeholder. Under enough concurrent load, two users signing up at roughly the same time could end up with the same generated placeholder — and one of the two signups would fail with a unique-constraint violation on the database.

The provisioning step now guarantees a unique domain per user with a stronger uniqueness strategy. New signups no longer trip over each other under load. Pretty rare bug, but a frustrating one to hit because it surfaced as "create account failed" with no obvious cause.