feat: setup logs retention by max file size

docs/api.md

@@ -45,9 +45,12 @@ Query: `q`, `actor` (`user:ID`, `key:ID`, `system`), `category` (`articles`, `me
 are UTC. The list answers `{ items, total, page, size }`, newest first (`created_at`, then `id`).
 
 `GET /v1/settings` also returns `activity_retention_days` (30, 90, 365 or 0 for forever, default
-90), `activity_last_cleanup_at` and `activity_last_cleanup_removed`. `PATCH /v1/settings` with
-`{"activity_retention_days": 30}` is session only and owner only (403 "Only the owner can do this.");
-any other value answers 400.
+90), `activity_retention_max_gb` (0 for no size limit), `activity_log_size_bytes`,
+`activity_last_cleanup_at` and `activity_last_cleanup_removed`. `PATCH /v1/settings` with
+`{"activity_retention_days": 30}`, `{"activity_retention_max_gb": 5}` or both is session only and
+owner only (403 "Only the owner can do this."); any other value answers 400. The daily cleanup
+removes events past the age limit, then the oldest events until the log fits the size limit.
+Whichever limit is reached first wins.
 
 ### What is recorded
 Every real action is recorded by its use case, once, after the change was made (outcome `success`)