Closes#2965.
hive-c0re auto-generates an OpenAPI 3 spec via utoipa
(hive-c0re/src/dashboard/mod.rs's ApiDoc), served raw at
/api/openapi.json and browsable as a Swagger UI at /api/docs, but
docs/ never mentioned either — genuinely zero hits grepping the whole
docs/ tree. Documented both in docs/web-ui/dashboard.md's Dashboard
endpoints + H0M3 page sections.
Also added the H0M3 hub's API tile mara suggested ("maybe also add
home page app that opens swagger ui") -- a plain static link to
/api/docs, no gating needed since the endpoint always exists
(unlike Forge/Matrix, which are conditionally enabled).
3f2fdeac70