Reports refused while report files cannot be written (closes #123)

A failed write of a report file used to drop its reports, and
POST /api/v1/reports kept answering 200 for as long as writes failed.
Now a failed write puts its reports back in the buffer, still counted
against DATA_DIR_MAX_BYTES, and Append refuses every report, which the
handler answers 500, until a write succeeds. The minute flush tries
the held reports again, so storing resumes with no restart once
writing works, and the buffer takes nothing new while writes fail.

Model: opus-5-5

TODO.md

@@ -22,6 +22,14 @@ Decide whether the repo moves to the layout `REPO_POLICIES.md` gives, with
 
 # Completed Steps
 
+- 2026-10-14: while report files cannot be written, `POST /api/v1/reports`
+  answers 500, and storing recovers once writing works again, with no restart
+  ([#123](https://git.eeqj.de/sneak/netwatch/issues/123)). A failed write puts
+  its reports back in the report buffer, still counted against
+  `DATA_DIR_MAX_BYTES`, where they used to be lost; the next flush, within a
+  minute, tries them again. Until a write succeeds, every report is refused and
+  nothing of it is kept, so the buffer does not grow while writes fail. A test
+  posts reports through the report handler before, during and after a failure
 - 2026-10-07: the files shared from `sneak/prompts` are its copies at commit
   `dd4027b` ([#113](https://git.eeqj.de/sneak/netwatch/issues/113)), with this
   repository's own entries after the shared content in `.gitignore`,