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>

server/internal/httputil/errors.go

@@ -81,6 +81,11 @@ type MatrixError struct {
 	RemoteStatus int    `json:"status,omitempty"`
 	RemoteBody   string `json:"body,omitempty"`
 
+	// DelayedErrCode and MaxDelayMS accompany a delayed event whose delay
+	// is longer than the server allows (MSC4140).
+	DelayedErrCode string `json:"org.matrix.msc4140.errcode,omitempty"`
+	MaxDelayMS     int64  `json:"org.matrix.msc4140.max_delay,omitempty"`
+
 	// status is the HTTP status to send; not part of the JSON body.
 	status int
 }