feat: setup logs retention by max file size

docs/design/screens/settings.md

@@ -184,11 +184,12 @@ Duplicates tab: secondary "Purge duplicates" button (merge) next to the sort men
 
 The last card on the page, below Media cleanup, owner only (hidden for admins and for the read-only Media cleanup view), same card styling and 720 px column as the others.
 
-- 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.
+- Header: "Retention" / "How long activity is kept, or how much of it. Whichever limit is reached first removes the oldest events." "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: "Events older than this are removed." when a size limit is set, otherwise "Older events are removed automatically." For Forever: "Nothing is removed automatically. The log grows over time." without a size limit, "No age limit. Only the size limit applies." with one.
+- An "or" divider, then "Keep at most" `USelectMenu` (160 px wide, full width on mobile): "No size limit" (default) and sizes in GB. Helper without a limit: "Optional. Logs can grow without a size cap." With one: "When logs pass 5 GB, the oldest events are removed. Currently 3.2 GB." The figure after "Currently" is the real size of the activity log. The size options are listed in the API note below.
+- Info alert (`lock`): "Activity logs are append-only", "Events can't be edited or deleted from the app. Only the retention limits remove 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 with no size limit: "Never" and "Not scheduled". Forever with a size limit still shows a schedule, because the size check runs at every cleanup.
+- Both selects save on change. Choosing a shorter age, or a smaller size limit than the current one, first shows a warning alert, "Events older than 30 days will be removed" (or "Events over 5 GB will be removed"), "This happens at the next cleanup on 7 Oct 2026, 03:00. It can't be undone.", buttons "Apply 30 days" (or "Apply 5 GB") and "Cancel"; nothing is sent until Apply. Lengthening, raising, choosing Forever or choosing "No size limit" 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.
+- States: "Default, 90 days or 5 GB", "Age only (no size limit)", "Size only (forever)", "Shortening the period" (also with a 5 GB limit), "Forever" and "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_retention_max_gb` (0 for no size limit, otherwise one of the size options), `activity_log_size_bytes`, `activity_last_cleanup_at` (null before the first cleanup) and `activity_last_cleanup_removed`. `PATCH /v1/settings` takes `{ "activity_retention_days": 30 }`, `{ "activity_retention_max_gb": 5 }` or both; 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, removes events older than the age limit, then removes the oldest events until the log is under the size limit, and never touches the last 24 hours.