Stop pinning usernames on admin Add (manual_username=false) so sync matches
invoice-created users. Add migration to reset manual_username for existing
rows so they re-enter ListForSync; explicit renames still pin via SetUsername.
- subscription_type defaults to lifetime when omitted; validated when provided
- years is only required (and enforced) when subscription_type is yearly
- username uniqueness check and validation are skipped when username is empty
- Update OpenAPI spec to reflect pubkey as the only required field
