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

README.md

@@ -559,9 +559,11 @@ What the [upaas](https://git.eeqj.de/sneak/upaas) app for netwatch needs:
   check answers 503 from a failed write of a report file until a write succeeds,
   and the backend tries the write again every minute. Three failed checks in a
   row make the container `unhealthy`, and one that passes makes it `healthy`
-  again. upaas reads the container's health 60 seconds after a deploy and fails
-  the deploy unless it is `healthy`. The container also stops when either
-  process exits.
+  again. upaas reads the container's health once, 60 seconds after a deploy, and
+  fails the deploy unless it is `healthy`. A container that turns `unhealthy`
+  after that is neither restarted nor reported by upaas, whose `unless-stopped`
+  restart policy ignores health; only `docker ps` and `docker inspect` show it.
+  The container also stops when either process exits.
 
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