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/engagement/templateSeeds.js

@@ -332,23 +332,29 @@ const SEEDS = [
   // single absent variable renders nothing, in both parts). A standalone event
   // has no `seriesName` and its line disappears rather than reading "Part of .".
   //
-  // **No `{{actionUrl}}` and no button, deliberately.** There is no public event
-  // page until Phase 14, so the six public triggers declare no `url` variable at
-  // all (see `coreTriggers.js`), and a button here would render as an inert grey
-  // label in every mail — worse than none, because it advertises a link the
-  // reader cannot follow. Phase 14 adds the variable and the block together.
+  // **The button arrived with the page it points at** (Phase 14a). Until then
+  // there was no public event page, the six public triggers declared no url
+  // variable, and a button here would have rendered as an inert grey label in
+  // every mail — worse than no button, because it advertises a link the reader
+  // cannot follow. `eventUrl` is optional and `email.button` drops itself when
+  // its url interpolates to nothing, so an event that is not public still mails
+  // correctly: the block disappears rather than degrading.
   {
     key: 'notify.event-started',
     name: 'Event starting',
     channel: 'email',
     protected: false,
-    seedVersion: 1,
+    // Bumped with the button. A deployment whose operator has not customized
+    // this template gets the new one; one that has is left alone and reported as
+    // stale, which is the whole mechanism.
+    seedVersion: 2,
     subject: '{{title}} is starting',
     variables: [
       { name: 'title', type: 'string', required: true, example: 'The Yew Invasion' },
       { name: 'summary', type: 'string', required: false, example: 'Orcish warbands are massing north of Yew.' },
       { name: 'seriesName', type: 'string', required: false, example: 'The Yew Campaign' },
       { name: 'startsAtLabel', type: 'string', required: false, example: 'Saturday 12 September at 8:00 pm (America/New_York)' },
+      { name: 'eventUrl', type: 'string', required: false, example: '/site/events/the-yew-invasion?run=3692' },
       { name: 'unsubscribeUrl', type: 'string', required: false, example: 'https://example.com/unsubscribe/abc123' },
     ],
     blocks: [
@@ -356,6 +362,7 @@ const SEEDS = [
       text('summary', '{{summary}}'),
       text('when', '{{startsAtLabel}}', { muted: true }),
       text('series', '{{seriesName}}', { muted: true }),
+      button('open', 'Read more', '{{eventUrl}}', 'Read more about it here:'),
       divider('rule'),
       button('unsub', 'Unsubscribe', '{{unsubscribeUrl}}', 'To stop these emails, use this link:'),
     ],