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

client/src/routes/admin/views/EventEditor.jsx

@@ -1164,11 +1164,24 @@ export default function EventEditor() {
           <span className="field-label">Storyline</span>
           <textarea className="input" rows={4} value={form.body} onChange={(e) => set({ body: e.target.value })} />
         </label>
+        {/* Announcement, not permission. Publishing is what makes an event
+            RUNNABLE, so without this switch a surprise invasion would have to be
+            advertised a fortnight in advance in order to be allowed to happen. */}
+        <label style={{ display: 'flex', alignItems: 'center', gap: 10, marginTop: 14 }}>
+          <input
+            type="checkbox"
+            checked={Boolean(form.listed)}
+            onChange={(e) => set({ listed: e.target.checked })}
+          />
+          <span className="field-label" style={{ margin: 0 }}>Show on the public calendar</span>
+        </label>
         <p className="sans dim" style={{ fontSize: '0.78rem', margin: '10px 0 0' }}>
           The grace window is how late this event may still start: past it an occurrence becomes
           <em> missed</em> rather than beginning hours after it was announced. Two runs sharing a
           concurrency key never overlap — <code>{'{placeholders}'}</code> are filled from the run&rsquo;s own
-          params.
+          params. An event that is <em>not</em> shown publicly still schedules, still runs and is
+          still on this calendar — it is simply not announced, and its lifecycle mails carry no
+          link because there is no page to link to.
         </p>
       </div>