Proxy outpost sessions live on local /dev/shm and break HA, restarts, and sockets

## What

proxy outpost stores sessions on local filesystem under /dev/shm, so HA replicas, restarts, and shm-full all drop sessions and return HTTP 400. Also: split-container /dev/shm outpost bugs: sessions written to shm, and the proxy dials /dev/shm/authentik.sock for OAuth code redemption. proxy-outpost session bug: cookie value does not match stored session (forward-auth 302). Likely the same in-memory/shm session backend as the HA group.

## Want

- Proxy outpost stores sessions on local filesystem under /dev/shm, so HA replicas, restarts,
- Shm-full all drop sessions
- Return HTTP 400
- Related split-container /dev/shm outpost bugs: sessions written to shm,
- The proxy dials /dev/shm/authentik.sock for OAuth code redemption
- Related proxy-outpost session bug: cookie value does not match stored session (forward-auth 302)
- Likely the same in-memory/shm session backend as the HA group

_Review — socket vs session store._

## Ideas this came from

- [#18098 Outpost Sessions w/ HA](https://github.com/goauthentik/authentik/issues/18098) — Describe your question/ I'm trying to deploy a proxy outpost on k8s w/ a multi-replica deployment. I've used the manual k8s proxy outpost from the documentation
- [#19605 Session cookie value differs from stored session data (both Embedded and External Proxy Outpost)](https://github.com/goauthentik/authentik/issues/19605) — The Authentik Proxy Outpost (both embedded and external) generates session cookies with values that differ from the actual stored session data. This causes forw
- [#19740 Proxy Outpost Restart causes broken redirect, 400 status code, Envoy](https://github.com/goauthentik/authentik/issues/19740) — The proxy outposts store sessions in memory. When restarted, the session data is lost, and it causes a redirect to https:// 2026.5.6)](https://github.com/goauthentik/authentik/issues/25076) — Summary After upgrading a split-container embedded-outpost deployment from 2025.12.1 to 2026.5.6, authentik-outpost-proxy fails to redeem OAuth2 authorization c

The issue names no files, tests, or entry points. Start by tracing the proxy outpost session store and split-container socket path, then verify that HA replicas, restarts, and a full /dev/shm do not lose sessions or return HTTP 400, while OAuth code redemption still works.

Written by the indexing model from the issue text.