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. The buffer keeps the files it may delete in a list, oldest first; a
file joins it 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 not yet written leave no room for it on their own, and then no
file is deleted. A file whose deletion fails keeps counting; one
already deleted by hand counts as freed.

Model: opus-5-5

README.md

@@ -212,7 +212,7 @@ 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