Delete the oldest report files to stay under the size cap (closes #54)

When a report would take the report files past DATA_DIR_MAX_BYTES,
reportbuf now deletes the oldest report files until it fits, and does
the same at start when files left by an earlier run are already past
it. A file joins the files that may be deleted only once it is
completely written, so a file still being written is never deleted. A
report is refused with 507 only when the reports waiting to be written
fill the cap on their own, and then no file is deleted. The reports of
a failed write stop counting, and the part of its file written is
removed. A file whose deletion fails keeps counting; one already
deleted by hand counts as freed.

Model: opus-5-5

backend/internal/handlers/report_test.go

@@ -125,9 +125,9 @@ func TestHandleReportStorageFailureIsNon2xx(t *testing.T) {
 	}
 }
 
-// TestHandleReportFullIs507 checks the answer when the report files
-// are at their size cap: 507 and the usual error body, which tells
-// the client nothing more.
+// TestHandleReportFullIs507 checks the answer when the reports waiting
+// to be written fill the size cap: 507 and the usual error body, which
+// tells the client nothing more.
 func TestHandleReportFullIs507(t *testing.T) {
 	t.Parallel()