feat: activity log backend foundation

docs/configuration.md

@@ -103,6 +103,10 @@ Relative paths are resolved against the process's working directory. In producti
 
 For example, `WorkingDirectory=/var/lib/renewcms` with `DB_FILE=./data/renewcms.db` puts the database in `/var/lib/renewcms/data`. Browser-update `--data-dir` must name that directory, because the app reads helper status and update records beside the database. It is not necessarily `WorkingDirectory` or `ReadWritePaths`. Do not change an existing `DB_FILE` merely to match the helper: that could open a different, empty database. See [browser-update setup](deployment.md#browser-updates).
 
+## Activity log retention
+
+The activity log needs no environment variable. Its retention period (30, 90, 365 days or forever, 90 by default) is a setting the owner changes with `PATCH /v1/settings`, and the daily cleanup runs at 03:00 in the server's time zone (`TZ`, or the host's zone).
+
 ## Adding a variable
 
 1. Read it in the code where it is needed (infrastructure or `api/`, never `internal/domain`).