docs: registration is closed (SSO/LLDAP only); note Draupnir encryption status

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

README.md

@@ -84,6 +84,7 @@ matrix/
 - Service: `draupnir.service`
 - Management room: `#management:matrix.lotusguild.org` (`!mEvR5fe3jMmzwd-FwNygD72OY_yu8H3UP_N-57oK7MI`)
 - Bot account: `@draupnir:matrix.lotusguild.org` (power level 100 in all protected rooms and the Lotus Guild space)
+- Encryption: off (`experimentalRustCrypto: false`), so Draupnir cannot read commands sent in the encrypted management room; send them unencrypted for now. Turning it on needs a device-bound access token first: with the current token, 2.9.0 exits with "Encryption not possible: server not revealing device ID" (tried and reverted 2026-10-16, #26 / #28).
 - Subscribed ban lists: `#community-moderation-effort-bl:neko.dev`, `#matrix-org-coc-bl:matrix.org`
 - Rebuild: `NODE_OPTIONS="--max_old_space_size=6144" npm run build`
 - Healthz endpoint: `http://10.10.10.24:8081/healthz` (200 = healthy, 418 = disconnected)
@@ -798,7 +799,7 @@ Leaving a space does **not** leave its child rooms; they are independent rooms w
 - [ ] `synchronous_commit = off` — **deliberately NOT applied.** Would be a real write-latency win, but this cluster hard-resets every few hours (HA self-fences on corosync quorum loss + the large1 DIMM), so it would drop up to ~600ms of committed messages *per reset* rather than once in a blue moon. Revisit once the cluster is stable.
 
 ### Auth & SSO
-- [x] Token-based registration
+- [x] Open registration disabled (`enable_registration: false`, 2026-10-16): accounts come from LLDAP and are created on first sign-in through Lotus SSO (Authelia OIDC, which keeps its own registration on). The old registration token was deleted.
 - [x] SSO/OIDC via Authelia
 - [x] `allow_existing_users: true` for linking accounts to SSO
 - [x] Password auth alongside SSO