Paginate the whole activity feed

Paginate the whole activity feed

Both activity pages stopped at a fixed number of events: the repository
feed was truncated at 100, and the instance feed merged 100 per
repository and then took 200 before its pager. Anything older was
unreachable.

Now that issues, changes, comments and state changes are index rows, the
whole feed is assembled from the database and paginated:

- the repository feed and the instance feed keep the whole sorted,
  filtered list, and both show 50 events per page;
- the repository feed gains a pager (it had none), and the instance
  pager now reaches all history;
- the kind/actor/assignee filters travel through the page links, so
  paging never widens the feed;
- the repository pager swaps the `#activity-feed` fragment over HTMX like
  the chips and the filter form, and the page links are plain hrefs when
  JavaScript is not there;
- the feed reads comment summaries (parent, id, author, created) rather
  than every comment body it will not render.

The home rail still asks for a bounded feed, so it keeps its handful of
events. The feed is assembled in memory per request, so a page costs
O(history) instead of O(page); that is the accepted trade for now, and a
SQL event stream is the follow-up if a large history makes it visible.