fix: add database constraint to enforce minimum username length (#19453)

Username length and format, via regex, are already enforced at the
application layer, but we have some code paths with database queries
where we could optimize away many of the DB query calls if we could be
sure at the database level that the username is never an empty string.

For example: https://github.com/coder/coder/pull/19395

---------

Signed-off-by: Callum Styan <callumstyan@gmail.com>

coderd/database/check_constraint.go

@@ -7,6 +7,7 @@ type CheckConstraint string
 // CheckConstraint enums.
 const (
 	CheckOneTimePasscodeSet                        CheckConstraint = "one_time_passcode_set"                            // users
+	CheckUsersUsernameMinLength                    CheckConstraint = "users_username_min_length"                        // users
 	CheckMaxProvisionerLogsLength                  CheckConstraint = "max_provisioner_logs_length"                      // provisioner_jobs
 	CheckValidationMonotonicOrder                  CheckConstraint = "validation_monotonic_order"                       // template_version_parameters
 	CheckUsageEventTypeCheck                       CheckConstraint = "usage_event_type_check"                           // usage_events