docs(v3): prepare final SaaS stabilization

* docs(v3): add final stabilization status

* docs(v3): add final release checklist

* docs(v3): add final release notes draft

* ci(v3): track final stabilization branch

docs(v3): prepare SaaS release candidate lot 15

Prepare v3.0-saas release candidate documentation.

Validated by CI before squash merge.

docs(release): add v2.0-ai release status

docs(release): add v1.0-pro changelog entry

docs(release): update changelog with v1.5.0 release preparation