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.

README.md

@@ -27,8 +27,15 @@ cloud control plane (`continuum-backend`).
 - **Cloud uplink** - a WebSocket client to `continuum-backend` that
   connects, sends a heartbeat on a timer, and reconnects on drop
   (`src/uplink/`).
-- **go2rtc watchdog** - restarts the camera-restreaming subprocess if it
-  dies (`src/go2rtc.rs`).
+- **Camera restreaming** - generates go2rtc's configuration from the camera
+  URLs the printers report, and restarts go2rtc when it changes
+  (`src/camera.rs` + `src/go2rtc.rs`). Only the X1/H2 class publishes an
+  RTSP URL: confirmed against two real machines, an H2C reports
+  `rtsps://<ip>:322/streaming/live/1` and a P1P reports a camera with no URL
+  for it. A P1/A1 camera needs Bambu's proprietary port-6000 JPEG stream,
+  which this crate does not speak, so those printers advertise no stream
+  rather than one that will not play. Note the stream is reachable on the
+  gateway's LAN only - there is no relay.
 - **Bambu file upload** - implicit FTPS (port 990) into the printer's own
   `cache/` directory (`src/printer/bambu_ftps.rs`, via
   `BambuPrinter::upload_file`). **Confirmed live against a P1P on
@@ -97,11 +104,12 @@ cargo run --example bambu_commands_demo    # round-trips hand-written structs ag
 
 ```
 src/
-  main.rs               Loads+connects the printer fleet, then runs the uplink and go2rtc watchdog side by side
+  main.rs               Loads+connects the printer fleet, then runs the uplink, camera config, watchdog and completion watcher side by side
   config.rs             Loads settings from environment variables
   fleet.rs              Loads printers.toml into ready-to-use PrinterHandles
   uplink/                WebSocket client to continuum-backend: connect, heartbeat, reconnect on drop
-  go2rtc.rs             Restarts the go2rtc camera-restreaming process if it dies
+  camera.rs             Writes go2rtc's config from the camera URLs the printers report
+  go2rtc.rs             Restarts the go2rtc camera-restreaming process if it dies, or when the config changes
   printer/
     mod.rs               GenericPrinter trait + PrinterBase + PrinterHandle enum - the trait+enum
                          "inheritance" pattern this project uses instead of class inheritance