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.
Tests also cover a report appended while a write that fails runs, and
the cap once a failed write's reports are written.

Model: opus-5-5

internal/handlers/report.go

@@ -93,7 +93,7 @@ func (s *Handlers) decodeErrorStatus(err error) int {
 
 // appendErrorStatus logs a failure to store a report and returns
 // the status to send: 507 when the reports waiting to be written fill
-// the size cap, otherwise 500.
+// the size cap, otherwise 500, as when report files cannot be written.
 func (s *Handlers) appendErrorStatus(err error) int {
 	if errors.Is(err, reportbuf.ErrFull) {
 		s.log.Warn("report refused: " +