refactor(sponsor-panel): namespace HTMX routes under /api/htmx/
Move the three HTMX form endpoints (invite, logo, thoth-token) behind
an /api/htmx/ prefix so they are visibly distinct from OAuth and page
routes. Updates handlers, dashboard template, godoc, and the endpoints
table in docs/README.md.
Assisted-by: Claude Opus 4.7 via Claude Code
Signed-off-by: Xe Iaso <me@xeiaso.net>