chore: groundwork for activity page

docs/design/screens/settings.md

@@ -177,3 +177,18 @@ Duplicates tab: secondary "Purge duplicates" button (merge) next to the sort men
 
 ### Components (Nuxt UI v4)
 `UCard`, `USwitch`, `UInputNumber` (min 1, max 365, suffix), `UFormField`, `UAlert`, `UButton`, `UModal`, `USkeleton`, `useToast`.
+
+## Activity retention
+
+**API:** GET /v1/settings · PATCH /v1/settings (owner only). Visual reference: `source/RenewCMS Screens Activity.dc.html`, group "Settings retention". The board draws the other cards as a dashed "Existing Settings cards" placeholder and gives the page a different subtitle ("Site-wide options. Only owners can change them."), which is not a decision to change the existing one.
+
+A third card below Media cleanup, owner only (hidden for admins), same card styling, 640 px at most as drawn.
+
+- Header: "Retention" / "How long activity is kept." "Saved" (check, success) at the right after a save, fading after 2 s.
+- "Keep activity for" `USelectMenu`: 30 days, 90 days, 365 days, Forever. Default 90 days. Helper: "Older events are removed automatically." For Forever: "Nothing is removed automatically. The log grows over time."
+- Info alert (`lock`): "Activity logs are append-only", "Events can't be edited or deleted from the app. Only the retention period removes them."
+- Divider, then read-only lines: "Last cleanup: 6 Oct 2026, 03:00 · 142 events removed" and "Next cleanup: 7 Oct 2026, 03:00". Forever: "Never" and "Not scheduled".
+- Saves on change. Choosing a shorter period than the current one first shows a warning alert, "Events older than 30 days will be removed", "This happens at the next cleanup on 7 Oct 2026, 03:00. It can't be undone.", buttons "Apply 30 days" and "Cancel"; nothing is sent until Apply. Lengthening or choosing Forever saves at once.
+- Save error alert: "Couldn't save the retention period", "Your previous setting is still in place.", "Try again". The control reverts.
+- States: Default (90 days, saved), Shortening the period, Forever, Save error. Loading and Offline follow the Media cleanup card.
+- `GET /v1/settings` adds `activity_retention_days` (30, 90, 365 or 0 for forever), `activity_last_cleanup_at` (null before the first cleanup) and `activity_last_cleanup_removed`. `PATCH /v1/settings` takes `{ "activity_retention_days": 30 }`; any other value answers 400 "The request cannot be processed due to a mismatch in the format of the body."; a non-owner gets 403 "Only the owner can do this.". The cleanup runs once a day at 03:00 server time and never touches the last 24 hours.