Stop orphaning go2rtc, serve cameras sooner, keep its config private

Three defects found by running a real WebRTC viewer through the whole
stack for the first time (continuum-backend/scripts/e2e-camera.sh).

1. go2rtc was orphaned on every ordinary stop. `kill_on_drop(true)` was
   set, but main handled no signals, so SIGTERM - what `kill`, `docker
   stop` and a service manager send - ended the process on the spot with
   no destructor run and the child was never dropped. The orphan keeps
   :1984 and :8555 bound, so the restarted proxy cannot start its own,
   and it keeps a printer's access code in memory. One had been running
   for 4.5 days. main now selects on SIGTERM/SIGINT so it returns
   normally and the supervisor is dropped. A SIGKILL or OOM kill still
   orphans it; PR_SET_PDEATHSIG would cover that but needs unsafe and a
   libc dependency, so it is left for a decision.

2. A camera was advertised before go2rtc could serve it. The comment on
   REBUILD_INTERVAL said the first useful rebuild is "a few seconds
   after startup", but a tokio interval ticks immediately - with the
   fleet still empty - and the next tick was 30s away. For that window
   the cloud offered a camera and go2rtc answered "stream not found".
   Polling every 5s costs nothing: it reads in-memory reports and only
   restarts go2rtc when the rendered config changes.

3. The generated go2rtc.yaml was created 0644 by tokio::fs::write. It
   holds the printer's access code and go2rtc's API password, so any
   local user could read both. It is now created owner-only and fresh,
   because a temp file left by a crash would otherwise keep its old
   mode. The new test pre-creates a stale 0644 temp file; against the
   old implementation it fails with mode 420 (0o644) where 384 (0o600)
   is expected.

cargo test: 193 lib tests and all integration tests pass.

src/camera.rs

@@ -55,11 +55,16 @@ use crate::uplink::SharedFleet;
 
 /// How often to rebuild the configuration from what the printers report.
 ///
-/// A camera URL changes when a printer's address changes, which is a DHCP
-/// lease event, not something that needs watching closely. The first useful
-/// rebuild is the one a few seconds after startup, once the first reports
-/// have arrived - before that the fleet has nothing to say.
-const REBUILD_INTERVAL: Duration = Duration::from_secs(30);
+/// The first tick fires immediately, before any printer has reported, so it
+/// always writes a config with no streams. The one that matters is the next,
+/// once the first reports have arrived - and until it runs, telemetry already
+/// advertises a camera that go2rtc cannot serve, so a viewer in that window
+/// gets "stream not found". At 30s that window was half a minute after every
+/// start (found by scripts/e2e-camera.sh, which asked at +7s).
+///
+/// Cheap to poll: it reads in-memory reports and compares the rendered
+/// config byte for byte, and go2rtc is only restarted when that changes.
+const REBUILD_INTERVAL: Duration = Duration::from_secs(5);
 
 /// The username every Bambu uses for its own services - MQTT, FTPS, and the
 /// camera. Not a guess: it is what `bambu_mqtt` and `bambu_ftps` already
@@ -162,8 +167,8 @@ impl ApiAuth {
     /// Generated once per run and then held, never re-derived: `render_config`
     /// is called every `REBUILD_INTERVAL` and its output is compared
     /// byte-for-byte to decide whether to restart go2rtc, so a password that
-    /// changed per call would restart it - and drop every live viewer - every
-    /// 30 seconds, forever.
+    /// changed per call would restart it - and drop every live viewer - on
+    /// every tick, forever.
     ///
     /// A generated password is never logged. It is written into the config
     /// file, which already holds every printer's access code and is already
@@ -369,8 +374,23 @@ pub async fn run(
 /// on the same filesystem, so a reader sees either the old file or the new
 /// one.
 async fn write_atomically(path: &str, contents: &str) -> std::io::Result<()> {
+    use tokio::io::AsyncWriteExt;
+
     let temp = format!("{path}.tmp");
-    tokio::fs::write(&temp, contents).await?;
+    // `tokio::fs::write` creates with 0666 minus the umask, so 0644: any local
+    // user could read the printer's access code and go2rtc's API password out
+    // of this file. Created owner-only instead, and created fresh - a temp file
+    // left behind by a crash would keep whatever mode it was given, because the
+    // mode applies only when the file is created.
+    let _ = tokio::fs::remove_file(&temp).await;
+    let mut options = tokio::fs::OpenOptions::new();
+    options.write(true).create_new(true);
+    #[cfg(unix)]
+    options.mode(0o600);
+    let mut file = options.open(&temp).await?;
+    file.write_all(contents.as_bytes()).await?;
+    file.flush().await?;
+    drop(file);
     tokio::fs::rename(&temp, path).await
 }
 
@@ -538,4 +558,29 @@ mod tests {
             "got:\n{config}"
         );
     }
+
+    /// The file holds a printer's access code and go2rtc's API password, so it
+    /// must not be readable by other local users. The second write matters: a
+    /// temp file left by a crash, created 0644, must not poison the next one.
+    #[cfg(unix)]
+    #[tokio::test]
+    async fn the_go2rtc_config_is_written_owner_only() {
+        use std::os::unix::fs::PermissionsExt;
+
+        let dir = std::env::temp_dir().join(format!("continuum-camera-{}", std::process::id()));
+        std::fs::create_dir_all(&dir).unwrap();
+        let path = dir.join("go2rtc.yaml");
+        let path_str = path.to_str().unwrap();
+
+        std::fs::write(format!("{path_str}.tmp"), "stale").unwrap();
+        std::fs::set_permissions(format!("{path_str}.tmp"), std::fs::Permissions::from_mode(0o644)).unwrap();
+
+        write_atomically(path_str, "secret: yes\n").await.unwrap();
+        assert_eq!(std::fs::read_to_string(&path).unwrap(), "secret: yes\n");
+        assert_eq!(std::fs::metadata(&path).unwrap().permissions().mode() & 0o777, 0o600);
+
+        write_atomically(path_str, "secret: again\n").await.unwrap();
+        assert_eq!(std::fs::metadata(&path).unwrap().permissions().mode() & 0o777, 0o600);
+        std::fs::remove_dir_all(&dir).unwrap();
+    }
 }