Health check answers 503 while report files cannot be written (closes #142)

The health check answered 200 "status":"ok" whatever happened, so a
container whose report files could not be written stayed healthy while
every report was refused with 500. The report buffer now says whether
its last write of a report file failed. While it has, the health
check's status is "error" and the handler answers 503 with the same
fields, which fails the image's HEALTHCHECK. At start no write has
failed, and the first write that succeeds brings back 200. A test
removes DATA_DIR to make a write fail and checks the 503, then the 200
once a write succeeds again.

Model: opus-5-5

TODO.md

@@ -21,6 +21,15 @@ Run `make frontend-viewport-test` in CI as its own step; it is not part of
 
 # Completed Steps
 
+- 2026-10-19: while report files cannot be written, the health check answers 503
+  with `"status":"error"` and the same fields, so the image's `HEALTHCHECK`
+  reports the container `unhealthy`
+  ([#142](https://git.eeqj.de/sneak/netwatch/issues/142)). It answered 200
+  `"status":"ok"` whatever happened, so the platform saw a healthy container
+  while every report was refused with 500. It answers 200 at start, before any
+  write has failed, and again from the first write that succeeds. A unit test
+  removes `DATA_DIR` to make a write fail and checks the 503, then puts it back
+  and checks the 200 after the next write
 - 2026-10-19: report file writes no longer overlap
   ([#150](https://git.eeqj.de/sneak/netwatch/issues/150)), so the reports of
   failed writes are written in the order they arrived. Two writes that failed at