Generate go2rtc's config from what the printers report

go2rtc has been supervised since before this commit, with nothing to
serve: `CONTINUUM_GO2RTC_CONFIG` pointed at `./go2rtc.yaml`, and that
file did not exist in the repository or anywhere else. A watchdog was
restarting a process that had no cameras configured.

`camera.rs` writes it, from the `rtsp_url` a printer publishes in its own
`print.push_status` `ipcam` block rather than from `printers.toml`. The
URL is already something the machine tells us, so asking an operator to
keep a second copy of it - and to update it by hand when a printer's
address changes - would be asking for drift.

Two tasks, not one: the builder decides what should be served, the
watchdog keeps the thing serving it alive. They meet at one `Notify`,
because go2rtc reads its configuration only at startup, so a changed
config needs a restart. The watchdog now selects the child's exit against
that notification and skips its backoff on a deliberate change.

What this does *not* cover is most printers, and that is the honest
limit. Only the X1/H2 class publishes an RTSP URL - confirmed against two
real machines: the H2C reports
`rtsps://192.168.0.21:322/streaming/live/1`, the P1P reports an `ipcam`
block with `ipcam_dev: "1"` and no URL at all. "A camera exists" and "a
camera you can connect to" are different facts. A P1/A1 needs Bambu's
proprietary port-6000 JPEG stream, which this crate does not speak, so
those printers report no `camera_stream` and a UI should offer a player
only when one is present.

Details worth keeping:

- The config is rendered sorted so an unchanged fleet is byte-identical,
  and on the first pass it is compared against the file on disk - without
  that, every daemon restart rewrites an identical file and drops every
  live viewer.
- Written through a `.tmp` and renamed. The watchdog can restart go2rtc at
  any moment, and a half-written config fails to parse, which takes every
  working stream down rather than one.
- Never logged, and gitignored: the credentials go in the URL because
  that is the only place go2rtc takes them for an RTSP source, so the file
  holds printer access codes.
- A reported URL with no scheme is refused rather than repaired. One bad
  entry makes go2rtc fail to load the whole file.
- No `rtsp:` or `webrtc:` block. The API listener is what serves WebRTC to
  a browser, which is the only consumer here; opening go2rtc's own RTSP
  server would publish every printer's camera on the LAN unauthenticated.
- `stream_base_url` is reported in `hello`, not derived from the listen
  address: `:1984` is a bind address and not something a browser can open.
  Unset means "advertise nothing", which is the right default, because a
  wrong URL makes the UI offer a player that cannot connect. It is a LAN
  address either way - there is no relay, so this works for an operator in
  the workshop and not for anyone outside it.

63 tests, clippy unchanged at the two deliberate variant-size warnings,
`cargo deny check licenses` ok. go2rtc itself stays operator-installed and
is not distributed.

tests/uplink_protocol.rs

@@ -185,6 +185,9 @@ fn empty_telemetry_fields_are_omitted_not_null() {
             bed_target: Some(0.0),
             print_error: Some(0),
             active_alerts: Some(0),
+            // The common case, and the third field that must be omitted: a
+            // P1P reports that it has a camera without giving a URL for it.
+            camera_stream: None,
         }],
     };
 
@@ -222,6 +225,7 @@ fn a_zero_stage_is_sent_rather_than_treated_as_absent() {
             bed_target: Some(80.0),
             print_error: Some(0),
             active_alerts: Some(0),
+            camera_stream: Some("p1p".into()),
         }],
     };
 
@@ -251,3 +255,33 @@ fn a_failed_command_result_carries_the_reason() {
     assert!(json.contains(r#""ok":false"#));
     assert!(json.contains("no printer with id 'nope'"));
 }
+
+/// A gateway with no configured stream base URL must omit the field, for the
+/// same reason the telemetry fields are omitted: the backend reads a missing
+/// key and an explicit null differently, and an unconfigured gateway is the
+/// default case, not an edge case.
+#[test]
+fn a_hello_without_a_stream_base_url_omits_it() {
+    let json = serde_json::to_string(&ClientMessage::Hello {
+        gateway_id: "gw-1".into(),
+        farm_id: "farm-1".into(),
+        token: "t".into(),
+        stream_base_url: None,
+    })
+    .expect("should serialize");
+    assert!(!json.contains("stream_base_url"), "got:\n  {json}");
+    // Without this the assertion above would also pass on an empty object.
+    assert!(json.contains(r#""gateway_id":"gw-1""#));
+}
+
+#[test]
+fn a_hello_carries_the_stream_base_url_when_one_is_configured() {
+    let json = serde_json::to_string(&ClientMessage::Hello {
+        gateway_id: "gw-1".into(),
+        farm_id: "farm-1".into(),
+        token: "t".into(),
+        stream_base_url: Some("http://gateway.local:1984".into()),
+    })
+    .expect("should serialize");
+    assert!(json.contains(r#""stream_base_url":"http://gateway.local:1984""#), "got:\n  {json}");
+}