Introduce a commercial license option alongside AGPL-3.0-only, require a
CLA for contributors, and document the terms in COMMERCIAL.md and
NOTICE. Add a script to stamp SPDX headers on Go files and apply it
across the tree.
Create Temporal billing sweep schedule, workflow and activities and
register them in the worker/start initialization. Add an HTMX preview
route and banner for plan switches (switch button now GETs a preview;
Confirm posts the switch). Extend the Stripe test mock to support
invoice previews and add integration tests for PreviewSwitch behavior.
cgalo5758