Voice and video calls: TURN endpoint and web client calls

Server: GET /voip/turnServer, configured under client_api.turn. A shared
secret (coturn use-auth-secret) gives each user expiring TURN REST API
credentials; static credentials are the fallback; with nothing configured
it answers {} so clients still try a direct connection.

Web client: 1:1 calls (Matrix VoIP via matrix-js-sdk) in rooms with one
other person. Voice and video buttons in the room header, a ringing card
with answer, answer without video and decline, and a call panel that
stays up across rooms: mute, camera, screen sharing, expand, hang up,
ring tones. Call events show in the timeline and room list. Works around
the SDK keeping a failed microphone request forever.

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

docs/configuration.md

@@ -62,6 +62,10 @@ List-valued variables are comma-separated. A single `-` clears a list:
 | `client_api.rate_limit.join.burst` / `.interval` | — | `10` / `1s` | The two join endpoints. |
 | `client_api.rate_limit.sync.burst` / `.interval` | — | `100` / `20ms` | `/sync` and the writes behind it: typing, receipts, to-device, presence. |
 | `client_api.rate_limit.media.burst` / `.interval` | — | `100` / `20ms` | Media downloads and thumbnails: a client opening a room asks for every avatar at once. Uploads are in the default bucket. |
+| `client_api.turn.uris` | `WARP_TURN_URIS` | `[]` | `turn:`, `turns:` and `stun:` URIs for calls. Empty = no relay; `/voip/turnServer` answers `{}`. |
+| `client_api.turn.shared_secret` | `WARP_TURN_SHARED_SECRET` | — | coturn's `static-auth-secret`: each user gets expiring TURN REST API credentials. |
+| `client_api.turn.user_lifetime` | `WARP_TURN_USER_LIFETIME` | `1h` | How long shared-secret credentials last. |
+| `client_api.turn.username` / `.password` | `WARP_TURN_USERNAME` / `WARP_TURN_PASSWORD` | — | Static credentials, used only without a shared secret. |
 | `sync.presence` | `WARP_SYNC_PRESENCE` | `false` | Off by default; see below. |
 | `sync.typing_timeout` | `WARP_SYNC_TYPING_TIMEOUT` | `30s` | How long a typing notification lasts, and the cap on what a client may ask for. |
 | `sync.long_poll_timeout` | `WARP_SYNC_LONG_POLL_TIMEOUT` | `30s` | The cap on a `/sync` `?timeout=`. |