feat: activity log backend foundation

docs/api.md

@@ -27,6 +27,28 @@ for example `https://cms.example/v1/article/`.
 | Users and invites | GET /v1/user/, PATCH /v1/user/me, POST /v1/user/invite (owner only), DELETE /v1/user/invite/{id} (owner only), POST /v1/user/invite/{id}/renew (owner only), GET /v1/auth/me |
 | Settings | GET /v1/settings |
 
+## Activity log
+The activity routes are session only: a key answers 403 "This action needs a real session." and a
+key never reads the audit trail. Events are append-only; there is no `POST`, `PUT`, `PATCH` or
+`DELETE` under `/v1/activity`.
+
+| Route | Who | Purpose |
+| --- | --- | --- |
+| `GET /v1/activity` | Any verified user | Filtered, paged events. Owner and admin see every event; any other role is scoped to its own events, and `actor` set to someone else answers 403 "Only owners and admins can do this." |
+| `GET /v1/activity/summary` | Owner, admin | Counts for the range, the previous period, active people and seven-bucket series. |
+| `GET /v1/activity/export` | Owner, admin | The same filters as the list without `page` or `size`, as a `text/csv; charset=utf-8` attachment (`when,actor,action,target,outcome,source,ip`). Cells starting with `=`, `+`, `-` or `@` get a leading quote. |
+
+Query: `q`, `actor` (`user:ID`, `key:ID`, `system`), `category` (`articles`, `media`, `users`,
+`api_keys`, `auth`, `system`), `outcome` (`all`, `success`, `failed`), `security` (`true`), `range`
+(`today`, `7d`, `30d`, `custom`; default `7d`), `from` and `to` (dates, with `custom`), `page` (from
+1) and `size` (25, 50 or 100). Any invalid value answers 400 with the body-mismatch message. Dates
+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.
+
 Article routes accept `?format=markdown|html|json` (default `markdown`). `GET /v1/media/` accepts
 `q`, `hash`, `cursor`, `filter`, `sort` and `limit`.