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>

internal/storage/events.go

@@ -78,12 +78,38 @@ func (r *EventRepo) Create(ctx context.Context, p CreateEventParams) (*domain.Ev
 		return nil, fmt.Errorf("seed owner collaborator: %w", err)
 	}
 
+	// Block F: auto-seed reminder messages so the host gets the
+	// "we'll nudge people for you" experience without lifting a finger.
+	// Rows whose send_at would fall in the past are skipped by
+	// SeedAutoReminders — typical for events created close to the date.
+	// Hosts can edit / cancel any of these from the Communications tab.
+	for _, m := range domain.SeedAutoReminders(ev.ID, ev.EventDate) {
+		if _, err := tx.Exec(ctx, `
+            INSERT INTO scheduled_messages
+                (event_id, send_at, audience, channel, template_key, subject, body, status)
+            VALUES ($1, $2, $3, $4, $5, $6, $7, $8)
+        `, m.EventID, m.SendAt, m.Audience, m.Channel,
+			m.TemplateKey, m.Subject, m.Body, m.Status); err != nil {
+			return nil, fmt.Errorf("seed auto-reminder %s: %w",
+				ifNilString(m.TemplateKey), err)
+		}
+	}
+
 	if err := tx.Commit(ctx); err != nil {
 		return nil, err
 	}
 	return ev, nil
 }
 
+// ifNilString is a tiny helper so the error message above stays readable
+// when an auto-reminder row somehow doesn't carry a template key.
+func ifNilString(p *string) string {
+	if p == nil {
+		return "<unknown>"
+	}
+	return *p
+}
+
 func (r *EventRepo) Get(ctx context.Context, id uuid.UUID) (*domain.Event, error) {
 	const q = `
         SELECT id, host_id, name, slug, event_date, venue, max_capacity, settings, status, created_at, updated_at, fraud_medium_threshold, fraud_high_threshold, fraud_block_threshold