Add JSON activities API endpoint (#2783)

* Add JSON events API endpoint

* Add regression test for event particulars defaults

* Move JSON events API to a dedicated ActivitiesController

The events endpoint served both the HTML day timeline and the JSON API
feed, but the two paths shared no data or behavior — the HTML side uses
DayTimelinesScoped while the JSON side built its own query. Splitting
into ActivitiesController gives the API its own home at
GET /:account/activities.json without dragging in the timeline
before_actions.

Also preloads comment creator in Event.preloaded to avoid an N+1 when
rendering comment eventables in the JSON feed.

app/views/activities/_activity.json.jbuilder

@@ -0,0 +1,21 @@
+json.(event, :id, :action)
+json.created_at event.created_at.utc
+json.description event.description_for(Current.user).to_plain_text
+json.particulars event.api_particulars
+json.url(
+  case event.eventable
+  when Comment then card_url(event.eventable.card, anchor: dom_id(event.eventable))
+  else polymorphic_url(event.eventable)
+  end
+)
+json.eventable_type event.eventable_type
+
+json.eventable do
+  case event.eventable
+  when Card    then json.partial! "cards/card", card: event.eventable
+  when Comment then json.partial! "cards/comments/comment", comment: event.eventable
+  end
+end
+
+json.board event.board, partial: "boards/board", as: :board
+json.creator event.creator, partial: "users/user", as: :user

app/views/activities/index.json.jbuilder

@@ -0,0 +1 @@
+json.array! @page.records, partial: "activities/activity", as: :event