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/clientapi/rtc.go

@@ -0,0 +1,51 @@
+package clientapi
+
+import (
+	"net/http"
+	"strings"
+
+	"github.com/dariuszjaniak/warp/server/internal/httputil"
+	"github.com/dariuszjaniak/warp/server/internal/userapi"
+)
+
+// rtcTransportsPath is MSC4143's endpoint for the group call backends a
+// client may use: the same list /.well-known/matrix/client carries as
+// org.matrix.msc4143.rtc_foci, behind authentication and without needing
+// the discovery document.
+const rtcTransportsPath = "/_matrix/client/unstable/org.matrix.msc4143/rtc/transports"
+
+// RTCTransport is one group call backend: a LiveKit SFU, named by the JWT
+// service that hands out tokens for it.
+type RTCTransport struct {
+	Type              string `json:"type"`
+	LiveKitServiceURL string `json:"livekit_service_url"`
+}
+
+// RTCTransports lists the configured group call backends, or nil for none.
+func RTCTransports(liveKitServiceURL string) []RTCTransport {
+	if liveKitServiceURL == "" {
+		return nil
+	}
+	return []RTCTransport{{Type: "livekit", LiveKitServiceURL: strings.TrimRight(liveKitServiceURL, "/")}}
+}
+
+type rtcTransportsResponse struct {
+	Transports []RTCTransport `json:"rtc_transports"`
+}
+
+// registerRTCRoutes adds the MSC4143 endpoint, if a backend is configured.
+// Without one it is left unregistered: the 404 is how a client learns there
+// is none.
+func (c *ClientAPI) registerRTCRoutes(r *httputil.Router) {
+	if len(c.rtcTransports) == 0 {
+		return
+	}
+	c.authed(r, "GET "+rtcTransportsPath, c.limitDefault, c.handleRTCTransports)
+}
+
+// handleRTCTransports implements
+// GET /_matrix/client/unstable/org.matrix.msc4143/rtc/transports.
+func (c *ClientAPI) handleRTCTransports(w http.ResponseWriter, _ *http.Request, _ userapi.Authenticated) error {
+	httputil.WriteJSON(w, http.StatusOK, rtcTransportsResponse{Transports: c.rtcTransports})
+	return nil
+}