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/engagement-triggers.json

@@ -11,7 +11,7 @@
       "subjectKey": "runId",
       "audience": "subscribers",
       "ceiling": "authenticated",
-      "version": 1,
+      "version": 2,
       "variables": [
         {
           "name": "runId",
@@ -54,6 +54,13 @@
           "required": true,
           "example": 4,
           "description": "How many phases the pinned version has in total."
+        },
+        {
+          "name": "eventUrl",
+          "type": "url",
+          "required": false,
+          "example": "/site/events/the-yew-invasion?run=3692",
+          "description": "The public page for this occurrence."
         }
       ]
     },
@@ -66,7 +73,7 @@
       "subjectKey": "runId",
       "audience": "subscribers",
       "ceiling": "authenticated",
-      "version": 1,
+      "version": 2,
       "variables": [
         {
           "name": "runId",
@@ -88,6 +95,13 @@
           "required": false,
           "example": "The shard is down for an emergency patch.",
           "description": "What the staff member gave as the reason, when they gave one."
+        },
+        {
+          "name": "eventUrl",
+          "type": "url",
+          "required": false,
+          "example": "/site/events/the-yew-invasion?run=3692",
+          "description": "The public page for this occurrence."
         }
       ]
     },
@@ -100,7 +114,7 @@
       "subjectKey": "runId",
       "audience": "subscribers",
       "ceiling": "authenticated",
-      "version": 1,
+      "version": 2,
       "variables": [
         {
           "name": "runId",
@@ -136,6 +150,13 @@
           "required": true,
           "example": 95,
           "description": "How long the run took, start to end, in whole minutes."
+        },
+        {
+          "name": "eventUrl",
+          "type": "url",
+          "required": false,
+          "example": "/site/events/the-yew-invasion?run=3692",
+          "description": "The public page for this occurrence."
         }
       ]
     },
@@ -148,7 +169,7 @@
       "subjectKey": "runId",
       "audience": "subscribers",
       "ceiling": "authenticated",
-      "version": 1,
+      "version": 2,
       "variables": [
         {
           "name": "runId",
@@ -163,6 +184,13 @@
           "required": true,
           "example": "The Yew Invasion",
           "description": "The event title."
+        },
+        {
+          "name": "eventUrl",
+          "type": "url",
+          "required": false,
+          "example": "/site/events/the-yew-invasion?run=3692",
+          "description": "The public page for this occurrence."
         }
       ]
     },
@@ -223,7 +251,7 @@
       "subjectKey": "runId",
       "audience": "subscribers",
       "ceiling": "authenticated",
-      "version": 1,
+      "version": 2,
       "variables": [
         {
           "name": "runId",
@@ -273,6 +301,13 @@
           "required": false,
           "example": "Saturday 12 September at 8:00 pm (America/New_York)",
           "description": "The start time written out in the shard-local zone, for a mail to read."
+        },
+        {
+          "name": "eventUrl",
+          "type": "url",
+          "required": false,
+          "example": "/site/events/the-yew-invasion?run=3692",
+          "description": "The public page for this occurrence."
         }
       ]
     },
@@ -285,7 +320,7 @@
       "subjectKey": "runId",
       "audience": "subscribers",
       "ceiling": "authenticated",
-      "version": 1,
+      "version": 2,
       "variables": [
         {
           "name": "runId",
@@ -335,6 +370,13 @@
           "required": false,
           "example": "Saturday 12 September at 8:00 pm (America/New_York)",
           "description": "The start time written out in the shard-local zone, for a mail to read."
+        },
+        {
+          "name": "eventUrl",
+          "type": "url",
+          "required": false,
+          "example": "/site/events/the-yew-invasion?run=3692",
+          "description": "The public page for this occurrence."
         }
       ]
     },