Critical signup fix plus incremental project slugs

Two issues in the signup flow that were causing failures on concurrent registrations:

1. Signup error: a race condition on the default-project creation step caused some new accounts to be created with no associated project. The hook now runs atomically with the account creation, and the account-creation transaction rolls back if the project creation fails.
2. Project slugs: default project slugs were generated with a non-deterministic strategy that could collide. The new strategy uses an incremental base36 sequence that mathematically excludes collisions under any realistic load.

Signup is now reliable under burst load and every new account lands with a project ready to go.