feat(events): the public calendar, event pages and participation history (Phase 14a)

The anonymous surface an event was always for: GET /public/events,
/public/events/:slug and /public/events/series/:slug, plus
GET /player/events/history, and the four screens over them.

Four org-lead decisions taken up front: split Phase 14 into 14a (website)
and 14b (the app); add a `listed` flag rather than letting `state` mean both
schedulable and announced; put the `events` capability string in the version
block rather than publishing core as a pseudo-module; and drop "venue" from
the spec rather than adding a field nothing had ever built.

`listed` is announcement, not permission. Publishing is what makes a
definition runnable, so without a separate flag a surprise event would have
to be advertised in order to be allowed to happen. It is a column, a switch
in Phase 13's editor, and three SQL predicates -- never a filter applied
after a read, which works exactly as well until the first caller that forgets.

The public shapes are a projection, and the projection is the security
boundary: nothing is spread, so a column added to event_runs next year does
not ride out through it. The spec, health, cleanup, claims, errors and
member_key are all absent by construction.

The six public event triggers gained `eventUrl` (version 1 -> 2), carrying
?run= because the page lives at the definition's slug while every trigger is
about one occurrence. notify.event-started gained the button, at seedVersion 2.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016wDDVXWMDz82WqE1i969r4

server/src/model/events/eventSeries.db.js

@@ -29,6 +29,11 @@ const getById = async (id) => {
   return row || null
 }
 
+const getBySlug = async (slug) => {
+  const [row] = await query(`${SELECT_LIST} WHERE s.slug = ?`, [slug])
+  return row || null
+}
+
 const exists = async (id) => {
   const [row] = await query('SELECT id FROM event_series WHERE id = ?', [id])
   return Boolean(row)
@@ -70,4 +75,4 @@ const update = (id, s) =>
  */
 const remove = (id) => query('DELETE FROM event_series WHERE id = ?', [id])
 
-module.exports = { list, getById, exists, slugTaken, insert, update, remove }
+module.exports = { list, getById, getBySlug, exists, slugTaken, insert, update, remove }