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

README.md

@@ -220,7 +220,7 @@ What the [upaas](https://git.eeqj.de/sneak/upaas) app for netwatch needs:
     - `REPORTS_PER_MINUTE`, default `60`: reports each client address may send a
       minute
     - `DATA_DIR_MAX_BYTES`, default `1073741824` (1 GiB): the most room the
-      report files may take
+      report files may take; the oldest are deleted to stay under it
     - `CORS_ALLOWED_ORIGINS`, default empty: other origins whose pages may call
       the API
     - `DEBUG`, default `false`: debug logging