feat: sync ephemeral transient messages

Sync telesrv 570ccf8 (feat(ephemeral): implement Layer 228 transient messages).

Skipped telesrv docs changes per public sync rules; normalized the public appearance seed label.

internal/store/memory/ephemeral_report.go

@@ -0,0 +1,53 @@
+package memory
+
+import (
+	"context"
+	"sync"
+
+	"telesrv/internal/domain"
+)
+
+type ephemeralReportKey struct {
+	reporterUserID int64
+	channelID      int64
+	messageID      int
+	option         string
+	commentHash    [32]byte
+}
+
+// EphemeralReportStore is the deterministic in-memory test implementation.
+type EphemeralReportStore struct {
+	mu      sync.Mutex
+	reports map[ephemeralReportKey]domain.EphemeralAbuseReport
+}
+
+func NewEphemeralReportStore() *EphemeralReportStore {
+	return &EphemeralReportStore{reports: make(map[ephemeralReportKey]domain.EphemeralAbuseReport)}
+}
+
+func (s *EphemeralReportStore) CreateEphemeralReport(_ context.Context, report domain.EphemeralAbuseReport) (bool, error) {
+	if err := report.Validate(); err != nil {
+		return false, err
+	}
+	key := ephemeralReportKey{
+		reporterUserID: report.ReporterUserID, channelID: report.Evidence.Peer.ID,
+		messageID: report.Evidence.MessageID, option: report.Option, commentHash: report.CommentHash,
+	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if _, exists := s.reports[key]; exists {
+		return false, nil
+	}
+	s.reports[key] = report
+	return true, nil
+}
+
+func (s *EphemeralReportStore) Reports() []domain.EphemeralAbuseReport {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	out := make([]domain.EphemeralAbuseReport, 0, len(s.reports))
+	for _, report := range s.reports {
+		out = append(out, report)
+	}
+	return out
+}