Prepare the next source release and backfill September release notes #488

GitHub has published 2026.09.13 and 2026.09.20, but CHANGELOG.md still stops at 2026.09.08. Backfill those releases from their immutable tags, then prepare the next source release from reviewed and merged main only. Keep self-hosting labelled Preview and leave the production distribution gates in #388 open.

• Backfill CHANGELOG.md and include configuration, migration and rollback notes, especially notification rollout and the removal of the unused module tables in chore(db): drop the module tables and the Plane mapping that filled them #480.
• Update README/operator docs and inspect current demo screenshots in both themes as required by docs/releases.md.
• Complete bun run verify, bun run docs:build, fresh hosted CI and review for the release preparation PR.
• Publish the selected UTC-date tag only after the release preparation is merged, then verify its commit, notes and source archives.

At the audit baseline, main 539e646 has green CI. Open PRs #448, #455, #479, #481, #483 and #485 have unresolved review or validation work; exclude them unless independently cleared and merged. #484 and #486 need their final review and CI before inclusion.