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

backend/README.md

@@ -140,6 +140,14 @@ toward `DATA_DIR_MAX_BYTES` from the next start, only if removing it fails too.
 Each file contains one JSON object per line, compressed with zstd. Files are
 created with `O_EXCL` to prevent overwrites.
 
+The reports of a failed write stay in memory, and the next flush, within a
+minute, tries again to write them. From the failed write until a write succeeds,
+`POST /api/v1/reports` refuses every report with 500 and keeps nothing of it, so
+the page keeps its samples and sends them again at its next report. Reports
+already answered 200 are lost only if the server stops before a write succeeds;
+the final flush at stop tries them too, and if it fails, the process exits
+non-zero.
+
 ### Report limits
 
 `POST /api/v1/reports` takes reports from anyone who can reach it, without
@@ -158,16 +166,16 @@ credentials, so it is bounded instead. Both refusals below answer with the same
   `X-RateLimit-Reset` headers.
 - **Size cap.** The report files in `DATA_DIR` may total at most
   `DATA_DIR_MAX_BYTES`, counting the files already there at start. Reports
-  waiting in memory count at their uncompressed size until they are written;
-  those lost to a failed write stop counting, and the part of its file written
-  is removed. When a report would take the total past the cap, the oldest report
-  files are deleted to make room, and each deletion is logged with the file's
-  name and size; a file still being written is never deleted. A report is
-  refused with 507, and nothing of it is stored, only when the reports waiting
-  to be written fill the cap on their own, and then no file is deleted. At
-  start, report files past the cap, as after lowering it, are deleted the same
-  way. So the cap is how much of the newest reports is kept: the default of 1
-  GiB is small enough for any host; set it to the space you can give `DATA_DIR`.
+  waiting in memory count at their uncompressed size until they are written,
+  those of a failed write included; the part of its file written is removed.
+  When a report would take the total past the cap, the oldest report files are
+  deleted to make room, and each deletion is logged with the file's name and
+  size; a file still being written is never deleted. A report is refused with
+  507, and nothing of it is stored, only when the reports waiting to be written
+  fill the cap on their own, and then no file is deleted. At start, report files
+  past the cap, as after lowering it, are deleted the same way. So the cap is
+  how much of the newest reports is kept: the default of 1 GiB is small enough
+  for any host; set it to the space you can give `DATA_DIR`.
 
 ### CORS