Group voice and video calls with MatrixRTC and LiveKit

Every call in the web client now goes through MatrixRTC with a LiveKit
SFU, the model Element Call and Element X use; the legacy 1:1 m.call.*
calls are gone. In a room of two the other person is rung and can answer
or decline; in a bigger room a banner offers to join the call. Calls show
a grid of participant tiles with mute, camera, screen sharing and leave,
and media is end-to-end encrypted in encrypted rooms.

Server:
- MSC4140 delayed events (send/state with a delay, plus the
  delayed_events list/restart/cancel/send API), stored and scheduled, so
  a member whose client drops off is taken out of the call.
- POST /user/{userId}/openid/request_token, which lk-jwt-service uses to
  hand out LiveKit tokens.
- org.matrix.msc4143.rtc_foci in /.well-known/matrix/client, configured
  under client_api.rtc.
- Room presets let members send call membership state.
- Migration 0014 for both SQL dialects.

Deploy: LiveKit and lk-jwt-service in compose (calls.yml) and the helm
chart. Docs: docs/calls.md, configuration and deployment.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/README.md

@@ -13,6 +13,7 @@
 | [federation.md](federation.md) | the Server-Server API: discovery, keys, taking in events, joins across servers, the outbound queue, the switch |
 | [media.md](media.md) | `internal/mediaapi`: uploads, downloads, thumbnails, other servers' media, where the files go |
 | [e2ee.md](e2ee.md) | `internal/keyserver`: device and one-time keys, cross-signing, device list tracking, key backup |
+| [calls.md](calls.md) | voice and video calls with MatrixRTC and LiveKit: ringing, delayed events, OpenID, the JWT service, deploying it |
 | [push.md](push.md) | `internal/pushrules` and `internal/pushserver`: push rules, notification counts, pushers, the push gateway client |
 | [appservices.md](appservices.md) | `internal/appservice`: registration files, namespaces, acting as users, delivering events to bridges |
 | [admin-api.md](admin-api.md) | `/_warp/admin/v1`: the operator API behind the admin panel |