Insert a minimal template row (all-zeros UUID) so it appears in both
team and admin template listings. Guard delete endpoints to prevent
removal of the minimal template.
Introduces internal/layout package for centralized path construction,
migrates templates from name-based TEXT primary keys to UUID PKs with
team-scoped directories (WRENN_DIR/images/teams/{team_id}/{template_id}).
The built-in minimal template uses sentinel zero UUIDs. Proto messages
carry team_id + template_id alongside deprecated template name field.
Team deletion now cleans up template files across all hosts.
