feat(PROJ-29): Tenant-Quotas & Usage-Limits vollständig implementiert

- storage/quota.go: SQL-Bug gefixt (emails.size → size_bytes, email_refs JOIN)
- tenantstore/quota.go: GetUsage nutzt jetzt email_refs JOIN für korrekte Tenant-Isolation
- smtpd: ErrQuotaExceeded → SMTP 452 statt 554 (MTA-retry statt permanent reject)
- admin_handlers: handleCreateUser prüft max_users-Quota → HTTP 402 bei Überschreitung
- quota_handlers: handleGetTenantUsage gibt jetzt warnings-Feld mit soft-limit-Prozenten zurück
- server.go: spec-konforme Alias-Route GET /api/admin/tenants/{id}/usage registriert

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/tenantstore/quota.go

@@ -48,14 +48,17 @@ func (s *Store) GetQuota(ctx context.Context, tenantID int64) (TenantQuota, erro
 }
 
 // GetUsage returns the current resource usage for a tenant.
+// Email count and storage bytes are aggregated via email_refs (tenant-aware dedup).
 func (s *Store) GetUsage(ctx context.Context, tenantID int64) (*TenantUsage, error) {
 	u := &TenantUsage{}
 
-	// Email count and storage bytes from emails table
+	// Email count and storage bytes via email_refs JOIN emails (correct tenant isolation).
+	// size_bytes is the column name in the emails table (not size).
 	err := s.pool.QueryRow(ctx, `
-		SELECT COUNT(*), COALESCE(SUM(size), 0)
-		FROM emails
-		WHERE tenant_id = $1
+		SELECT COUNT(r.id), COALESCE(SUM(e.size_bytes), 0)
+		FROM email_refs r
+		JOIN emails e ON e.id = r.email_id
+		WHERE r.tenant_id = $1
 	`, tenantID).Scan(&u.EmailCount, &u.StorageBytes)
 	if err != nil {
 		return nil, fmt.Errorf("tenantstore: get usage emails: %w", err)