Live view over WebRTC (openpanel L1, server and browser)

A device reporting a `live` port streams its camera to the browser's own
RTCPeerConnection. The flows socket carries the signalling as text, so it
crosses the portal: `live` messages on a person's socket, `signal` on a
device's (start, sdp, candidate, bye, busy). The server routes by session,
holds a device to two viewers, ends a session when either socket closes or
no answer comes within 10 s, and gives each socket an outbox of its own
that ends a session rather than drop a message. TURN credentials are
minted per session in coturn's REST scheme (STUN_URLS, TURN_URLS,
TURN_SECRET), the viewer's with the device's offer; GET /devices/{id}/ice
hands them out too. The tunnel keeps a stream's early frames instead of
dropping them past eight and forgets them when the stream or link goes.

A media tile on a device's camera, and the Devices tab's preview, play
live when tapped, pause frames meanwhile, and fall back to frames with a
short note on busy, failure or no picture within 10 s. coturn is an
optional compose service behind the `turn` profile.

docs/interface/dashboards.md

@@ -208,7 +208,8 @@ at, and stops when it scrolls away, the tab is hidden or the tile closes. A
 clip keeps playing either way. One page is pushed frames for at most 8
 messages at once. A ninth tile fetches its frames one by one and does not turn
 a camera on. A [device's camera](#a-devices-camera) sends only while a tile like
-this wants it.
+this wants it. A tile on a device's camera that streams live shows **Live**:
+tap it for [live video](#live-view).
 
 For continuous video, set **Live stream** to a browser-compatible camera URL.
 The browser must be able to reach that URL; enrollment does not expose the
@@ -600,7 +601,7 @@ keeps the switch as it was set.
 
 ### A device's camera
 
-A camera device (openpanel on a NANO) reports four ports:
+A camera device (openpanel on a NANO) reports these ports:
 
 | Port | Type | Settings | What it sends |
 |---|---|---|---|
@@ -608,6 +609,7 @@ A camera device (openpanel on a NANO) reports four ports:
 | `person` | `bool` | `hold_s` (10) | whether someone is in view, held for `hold_s` after they leave |
 | `detections` | `json` | | `{count, boxes: [{score, x0, y0, x1, y1}]}`, boxes scaled 0 to 1, on change and at most once a second |
 | `snapshot` | `image` | | one stored frame each time `person` turns true |
+| `live` | `video`, on demand | | H.264 over WebRTC while a [live view](#live-view) runs; never wired in a flow |
 
 `camera` sends only while it has demand: while a page wants its frames (a
 dashboard tile, the preview on the device's page, a screen through the
@@ -633,6 +635,24 @@ A device may fetch the last 8 artifacts each of its messages pointed at, not
 only the current one, so a screen fetching a camera frame by frame through the
 portal is not refused a frame the camera has just replaced.
 
+#### Live view
+
+A media tile showing a device's `camera`, and the preview on the device's
+page, offer **Live** while the device is online and reports `live`. Tap the
+tile and the browser connects to the device with WebRTC, without a plugin,
+and plays its video in the tile under a **Live** badge. Tap again to stop. It
+stops by itself when the tile scrolls away, the tab is hidden or the page
+closes. While it plays the tile asks for no frames, so the camera encodes one
+picture rather than two, and frames come back when it ends.
+
+A device serves 2 live viewers. A third tile, a failed connection or no
+picture within 10 seconds stays on frames and says so briefly. On one network
+the two ends find each other directly. Between networks they need STUN and,
+behind strict NATs, a TURN relay: set `STUN_URLS`, `TURN_URLS` and
+`TURN_SECRET` (see [Configuration](../reference/configuration.md#live-view)).
+The server only passes on the few messages that set a connection up, the
+video never crosses it. A wall panel's own screen does not offer live view.
+
 ### Before device nodes
 
 Before device nodes a device carried its ports itself. On its first start an