* fix: enforce unique usernames
Registration checked for a taken name, hashed the password, then
inserted, with nothing in the schema enforcing uniqueness. Concurrent
sign-ups could create two accounts with one name, after which login,
password reset, proxy login and admin lookups by username picked
between them arbitrarily.
Add a unique index on users.username for every dialect. Existing
duplicates are renamed (later rows get a -<id> suffix, the oldest keeps
the name) rather than deleted. Registration and admin user creation now
return the same 409 when the insert loses the race.
* fix: avoid username collisions during migration
