feat(events): the minimal admin surface (Phase 3)

Three screens, an Events nav group and the six live run controls Phase 1 left
absent on purpose because nothing was in flight. An admin can now author,
publish, start and watch an event that announces things and cues a human; a
moderator can stop one that is going wrong.

Six controls, not eight. `advance` is absent because a phase today advances when
its steps go terminal — the per-step skip already does that — and Phase 5 is what
gives a phase an advance condition. Cancel takes `{ reason }`, not `{ cleanup }`,
until Phase 8's ledger exists. Every control is a compare-and-set on the status it
may act from, so a console rendered thirty seconds ago cannot act on a run that
has moved.

Fixes a defect in the Phase 2 runner: `advanceRun` drained up to
EVENT_STEPS_PER_TICK steps while only checking the run's status at the top of the
tick, so a pause pressed mid-batch did nothing for up to 24 more steps.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T6t8mrAWhZU5vnyYgZTMtL

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

@@ -23,8 +23,17 @@ const hydrate = (row) => row && { ...row, params: parseJson(row.params, null), r
 // only if every path a run can take reaches one of them.
 const TERMINAL = ['completed', 'cancelled', 'failed', 'missed']
 
+// `waiting_steps` is the count of PARKED steps: `running` with a NULL lease, the
+// pair `park()` alone produces, which means a cue waiting on a human. It is a
+// correlated subquery on an admin list bounded at 500 rows rather than a column,
+// because it is derived from the steps and a column would be a second writer's
+// opinion of them. It earns its cost on the list screen: a cue nobody notices is
+// a run that never advances, and the run itself looks perfectly healthy until
+// somebody opens it.
 const SELECT_LIST = `
-  SELECT r.*, d.title AS definition_title, d.slug AS definition_slug, v.version AS version_number
+  SELECT r.*, d.title AS definition_title, d.slug AS definition_slug, v.version AS version_number,
+         (SELECT COUNT(*) FROM event_run_steps s
+           WHERE s.run_id = r.id AND s.status = 'running' AND s.claim_expires_at IS NULL) AS waiting_steps
     FROM event_runs r
     JOIN event_definitions d ON d.id = r.definition_id
     JOIN event_versions    v ON v.id = r.version_id
@@ -241,6 +250,20 @@ async function transition(id, from, to, { phase, error, clearClaim = false } = {
   return Number(result?.affectedRows || 0) === 1
 }
 
+/**
+ * Just this run's status, for a caller that must not act on a stale read.
+ *
+ * The runner drains a bounded batch of steps from one run inside a single tick,
+ * and Phase 3 put a pause and a cancel button in a human's hand — so between two
+ * steps of that batch the run may have stopped. A loop that only re-checked at
+ * the top of the tick would answer a pause by dispatching another two dozen
+ * steps, which is not a pause. One column, by primary key.
+ */
+const statusOf = async (id) => {
+  const [row] = await query('SELECT status FROM event_runs WHERE id = ?', [id])
+  return row?.status || null
+}
+
 /**
  * Set health without touching status (§E).
  *
@@ -354,6 +377,7 @@ module.exports = {
   claimStart,
   claimTick,
   releaseClaim,
+  statusOf,
   transition,
   setHealth,
   concurrencyHolder,