Renames the stack to drop the _dev suffix now that it's production-ready.
Bumps AUTHENTIK_TAG from 2025.10.1 to 2026.8.3, adds the redis service
required by the authentik server/worker (was missing entirely), and
fixes stale/duplicated lines in the README.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>