Add sort_order to billing.plan_ladders and a migration.
Reshape models, queries, handlers and templates to group plans by
plan ladder (service axis) in rank order. Track the current rung per
ladder and render per-tier move affordances; only the free→paid path
is enabled, other moves render disabled with an explanatory reason.
Include operator sort_order edit, tests, docs and image assets.
Christian Galo
c409da9df2