Signup error fixed, username collisions handled cleanly

Signup was occasionally failing with an opaque error when two users tried to register at the same time with similar names (the auto-derived username would collide on the unique index). The new flow detects the collision before the insert and appends a deterministic numeric suffix.

"sarah-cole" already taken? You get "sarah-cole-2", and the user never sees the underlying error. The username trigger that handles this is documented in the DB scripts so the rule is reapplicable across environments.