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, at its place by name,
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/reportbuf/export_test.go

@@ -1,6 +1,9 @@
 package reportbuf
 
-import "time"
+import (
+	"os"
+	"time"
+)
 
 // FlushSizeThreshold exposes the buffer size at which Append starts
 // writing a report file to the external tests.
@@ -17,3 +20,10 @@ func (b *Buffer) Flush() error {
 func (b *Buffer) StopClock(at time.Time) {
 	b.now = func() time.Time { return at }
 }
+
+// OnFileCreated makes the buffer call fn with each report file it
+// writes from now on, once the file is created and before anything is
+// written to it.
+func (b *Buffer) OnFileCreated(fn func(f *os.File)) {
+	b.fileCreated = fn
+}