feat(tier2): reminders + broadcasts pipeline — Block F

The Communications surface. Hosts can schedule custom broadcasts to a
chosen audience (everyone / attending / pending / declined / maybe),
edit or cancel anything that hasn't fired, and review delivery
outcomes. Four auto-reminders are pre-seeded on every new event:
7-day, 3-day last call, 1-day, and day-of.

Schema (migration 0012)
- scheduled_messages — one row per message envelope, with status
  walking draft -> scheduled -> sending -> sent (or cancelled/failed).
  Partial index on (send_at) WHERE status='scheduled' for the
  scheduler poll; per-event index for the Communications tab list.
- message_deliveries — per-recipient outcomes so a partial-failure
  batch doesn't lose the rows that did succeed.

Domain
- MessageAudience / MessageChannel / MessageStatus enums
- SeedAutoReminders helper that returns four canonical reminder rows
  for a given event_date, skipping any whose send_at would land in
  the past (events created close to the date)

Storage
- MessageRepo: Create / CreateBatch / Get / ListByEvent / Update
  (locks the row and refuses unless status is draft|scheduled) /
  Cancel / PromoteToScheduled (the send-now path) / ListDue /
  ClaimForSending (atomic guard against two replicas double-sending) /
  MarkSent / MarkFailed / RecordDelivery / DeliveryStats /
  LoadRecipients (audience-filtered guest list) / CountRecipients
- EventRepo.Create now seeds auto-reminders in the same transaction
  that inserts the event and its owner collaborator row

API (all editor+, except recipient-count which is viewer+)
- GET    /events/{id}/messages
- GET    /events/{id}/messages/recipient-count?audience=...
- POST   /events/{id}/messages   (draft / schedule / send-now)
- PATCH  /events/{id}/messages/{message_id}
- POST   /events/{id}/messages/{message_id}/send-now
- DELETE /events/{id}/messages/{message_id}

Scheduler worker (cmd/notifier)
- New file scheduler.go: polls ListDue every 30s, claims each row
  atomically (ClaimForSending uses a status=scheduled guard so two
  notifier replicas don't double-send), renders subject and body
  per recipient with the {{guest_name}} / {{event_name}} /
  {{event_date}} / {{venue}} / {{rsvp_link}} placeholders, sends via
  the existing GuestEmailDispatcher (Resend > SMTP > SES > log
  stub, same picker as the API), records each delivery row.

Frontend
- New CommunicationsCard.vue with compose form (audience + channel +
  subject + body + send-mode radios), live "X guests will receive
  this" recipient-count preview, and three sub-tabs for Scheduled /
  Sent / Cancelled. Per-message Send-now and Cancel actions for
  draft/scheduled rows. Friendly labels for auto-seeded reminders
  ("1-day reminder", "Day-of reminder") so the slugs never leak.
- New top-level tab "Communications" on the event-detail page,
  between Collaborators and Branding.

Tests
- TestAutoReminderSeeding confirms a future-dated event lands the
  four canonical reminders in scheduled state.
- TestComposeAndEditMessage walks draft -> patch -> send-now ->
  cancel and asserts the conflict on PATCH-after-cancel.
- TestRecipientCountAudienceFilter seeds a known guest mix and
  checks every audience preset returns the right count.
- Full integration suite passes (~177s).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/pages/dashboard/events/[id].vue

@@ -69,8 +69,8 @@ const loading = ref(true)
 // (Collaborators + Branding, configured once) → Analytics (results,
 // checked periodically). The two action-y tabs anchor the ends; setup
 // clusters in the middle.
-type EventTab = 'guests' | 'collaborators' | 'branding' | 'analytics' | 'gate'
-const validTabs: EventTab[] = ['guests', 'collaborators', 'branding', 'analytics', 'gate']
+type EventTab = 'guests' | 'collaborators' | 'communications' | 'branding' | 'analytics' | 'gate'
+const validTabs: EventTab[] = ['guests', 'collaborators', 'communications', 'branding', 'analytics', 'gate']
 function tabFromHash(): EventTab {
   if (import.meta.client) {
     const h = window.location.hash.replace('#', '') as EventTab
@@ -783,11 +783,12 @@ function checkLabel(band?: string): string {
     >
       <button
         v-for="t in [
-          { id: 'guests',        label: 'Guests' },
-          { id: 'collaborators', label: 'Collaborators' },
-          { id: 'branding',      label: 'Branding' },
-          { id: 'analytics',     label: 'Analytics' },
-          { id: 'gate',          label: 'Gate' },
+          { id: 'guests',         label: 'Guests' },
+          { id: 'collaborators',  label: 'Collaborators' },
+          { id: 'communications', label: 'Communications' },
+          { id: 'branding',       label: 'Branding' },
+          { id: 'analytics',      label: 'Analytics' },
+          { id: 'gate',           label: 'Gate' },
         ] as { id: EventTab, label: string }[]"
         :key="t.id"
         role="tab"
@@ -1212,6 +1213,11 @@ function checkLabel(band?: string): string {
       <TeamCard :event-id="eventId" :your-role="event.your_role" />
     </div>
 
+    <!-- Communications (Tier 2 Block F). Reminders + custom broadcasts. -->
+    <div v-if="activeTab === 'communications' && event" class="mt-2">
+      <CommunicationsCard :event-id="eventId" :your-role="event.your_role" />
+    </div>
+
     <!-- Gate (Tier 2 Block G). The user-facing rebrand of the fraud
          detector: strictness presets + trusted networks + decision
          history, with the technical sliders/CIDR jargon tucked behind