admin: invite links without a username

Leaving the username empty when creating a user makes the account
invite-<hex>; the invitee picks the real username on the invite page
(taken names and the invite- prefix are refused). Without an email this
gives a plain invite link to share.

Assisted-by: Claude:opus-5

db/migrations/011_users_choose_username.rb

@@ -0,0 +1,7 @@
+Sequel.migration do
+  change do
+    # Created as "invite-<hex>"; the invitee picks the real username when
+    # accepting the invite.
+    add_column :users, :choose_username, TrueClass, null: false, default: false
+  end
+end