feat: activity log producers

docs/api.md

@@ -49,6 +49,40 @@ are UTC. The list answers `{ items, total, page, size }`, newest first (`created
 `{"activity_retention_days": 30}` is session only and owner only (403 "Only the owner can do this.");
 any other value answers 400.
 
+### What is recorded
+Every real action is recorded by its use case, once, after the change was made (outcome `success`)
+or after the use case refused it (outcome `failed`, with a plain `failure_reason`). Nothing is
+recorded for a read (no `GET`, no list, no public article read, no status poll), for a request that
+fails validation, or for a target that does not exist (404). A failed audit write is logged and
+never changes the response or the change itself.
+
+| Category | Actions |
+| --- | --- |
+| `articles` | `article.create`, `article.edit`, `article.publish`, `article.unpublish`, `article.delete` |
+| `media` | `media.upload`, `media.rename`, `media.delete`, `media.merge`, `media.purge` |
+| `users` | `user.register`, `user.verify`, `user.update_profile`, `invite.create`, `invite.cancel`, `invite.renew`, `invite.accept` |
+| `system` | `system.media_purge` (scheduled), `system.update_checked`, `system.update_downloaded`, `system.restart_requested`, `system.activity_cleanup`, `system.activity_retention_changed` |
+
+Target types: `article`, `image`, `user`, `invite`, `api_key` and `release` (id and name are the tag).
+Only the fields `title`, `online`, `body`, `filename` and `display_name` appear in `changes`, with the
+kinds `text` and `body`; a body edit carries a signed character count delta and never the text. A
+change that leaves a value as it was (same body, same publish state, same file name, same display
+name) records nothing. Params: `media.merge` and `media.purge` and `system.media_purge` carry
+`merged` and `articles_updated`, plus `failed_groups` when a group failed (still outcome `success`);
+`system.update_checked` carries `update_available`; `system.restart_requested` carries `kind` and,
+for an update, `tag` (it has no target). `system.update_downloaded` is recorded when the job reaches
+`ready` or `failed`, with the origin captured at request time; a refusal at the start is recorded
+immediately. A failed registration or invite acceptance has an `anonymous` actor named by the
+attempted username.
+
+`failure_reason` is one fixed sentence per domain error, matched with `errors.Is`, at most 255
+characters, for example "The title must be between 3 and 200 characters.", "That file name is not
+valid.", "The merge request is not valid.", "A purge is already running.", "Registration is closed
+on this site.", "That invitation has expired.", "The release feed could not be reached." or "The
+update could not be downloaded.". Any other error becomes "Something went wrong on our side. Please
+try again."; the text of the error never reaches the event, the API or the CSV. Passwords, hashes,
+codes, article bodies, file bytes, storage paths and asset urls are never recorded.
+
 Article routes accept `?format=markdown|html|json` (default `markdown`). `GET /v1/media/` accepts
 `q`, `hash`, `cursor`, `filter`, `sort` and `limit`.