feat: reproduce NetFlow collectors + force user-data replacement (INFRA-12)

- Build nfdump 1.6.23 from source in user-data (rrdtool-devel for librrd;
  not packaged on AL2023) and run nfcapd collectors as systemd units:
  nfcapd.service (Ronkonkoma udp/2055), nfcapd-locust.service (Locust udp/2056),
  + netflow-retention.timer (30d sweep). 1 GiB swapfile for build headroom.
- userDataCausesReplacement: true — a user-data change must actually re-run,
  so force instance replacement (stateless box, EIP re-associates).

Validated: build + all services active, listeners on 514/2055/2056.

lib/syslog-server-stack.ts

@@ -84,6 +84,15 @@ export class SyslogServerStack extends cdk.Stack {
     userData.addCommands(
       "set -euxo pipefail",
       "",
+      "# ── 1 GiB swap (build headroom + stability on the 512 MiB t4g.nano) ──",
+      "if [ ! -f /swapfile ]; then",
+      "  fallocate -l 1G /swapfile || dd if=/dev/zero of=/swapfile bs=1M count=1024",
+      "  chmod 600 /swapfile",
+      "  mkswap /swapfile",
+      "  echo '/swapfile none swap sw 0 0' >> /etc/fstab",
+      "fi",
+      "swapon -a || true",
+      "",
       "# ── rsyslog: listen on UDP/TCP 514 ──",
       "dnf install -y rsyslog",
       "cat > /etc/rsyslog.d/10-listen.conf <<'EOF'",
@@ -131,6 +140,74 @@ export class SyslogServerStack extends cdk.Stack {
       "  -a fetch-config -m ec2 \\",
       "  -c file:/opt/aws/amazon-cloudwatch-agent/etc/amazon-cloudwatch-agent.json -s",
       "systemctl enable amazon-cloudwatch-agent",
+      "",
+      "# ── NetFlow/IPFIX collectors (nfcapd) ──",
+      "# nfdump is not packaged for AL2023; build 1.6.23 from source (needs",
+      "# rrdtool-devel for librrd). Reconstructed under IaC for INFRA-12 — the",
+      "# original instance ran these as hand-installed systemd units. Captures",
+      "# are local-only (no consumer/shipping today); 30-day retention enforced.",
+      "dnf install -y gcc gcc-c++ make automake autoconf libtool flex bison libpcap-devel zlib-devel bzip2-devel rrdtool-devel tar",
+      "NFVER=1.6.23",
+      "curl -sfL https://github.com/phaag/nfdump/archive/refs/tags/v${NFVER}.tar.gz | tar xz -C /tmp",
+      "( cd /tmp/nfdump-${NFVER} && ./autogen.sh && ./configure && make -j1 && make install )",
+      "ldconfig",
+      "",
+      "mkdir -p /var/log/netflow/ronkonkoma /var/log/netflow/locust",
+      "chown -R ec2-user:ec2-user /var/log/netflow",
+      "",
+      "# Ronkonkoma gateway -> UDP 2055",
+      "cat > /etc/systemd/system/nfcapd.service <<'EOF'",
+      "[Unit]",
+      "Description=nfcapd NetFlow collector (Ronkonkoma, udp/2055)",
+      "After=network.target",
+      "[Service]",
+      "Type=simple",
+      "User=ec2-user",
+      "ExecStart=/usr/local/bin/nfcapd -p 2055 -l /var/log/netflow/ronkonkoma",
+      "Restart=always",
+      "[Install]",
+      "WantedBy=multi-user.target",
+      "EOF",
+      "",
+      "# Locust Ave gateway -> UDP 2056",
+      "cat > /etc/systemd/system/nfcapd-locust.service <<'EOF'",
+      "[Unit]",
+      "Description=nfcapd NetFlow collector (Locust Ave, udp/2056)",
+      "After=network.target",
+      "[Service]",
+      "Type=simple",
+      "User=ec2-user",
+      "ExecStart=/usr/local/bin/nfcapd -p 2056 -l /var/log/netflow/locust",
+      "Restart=always",
+      "[Install]",
+      "WantedBy=multi-user.target",
+      "EOF",
+      "",
+      "# 30-day retention sweep (daily 03:30 UTC)",
+      "cat > /usr/local/sbin/netflow-retention.sh <<'EOF'",
+      "#!/bin/bash",
+      "find /var/log/netflow -type f -name 'nfcapd.*' -mtime +30 -delete",
+      "EOF",
+      "chmod +x /usr/local/sbin/netflow-retention.sh",
+      "cat > /etc/systemd/system/netflow-retention.service <<'EOF'",
+      "[Unit]",
+      "Description=Delete NetFlow captures older than 30 days",
+      "[Service]",
+      "Type=oneshot",
+      "ExecStart=/usr/local/sbin/netflow-retention.sh",
+      "EOF",
+      "cat > /etc/systemd/system/netflow-retention.timer <<'EOF'",
+      "[Unit]",
+      "Description=Daily NetFlow retention sweep",
+      "[Timer]",
+      "OnCalendar=*-*-* 03:30:00 UTC",
+      "Persistent=true",
+      "[Install]",
+      "WantedBy=timers.target",
+      "EOF",
+      "",
+      "systemctl daemon-reload",
+      "systemctl enable --now nfcapd.service nfcapd-locust.service netflow-retention.timer",
     );
 
     const instance = new ec2.Instance(this, "Instance", {
@@ -148,6 +225,9 @@ export class SyslogServerStack extends cdk.Stack {
       securityGroup: sg,
       role,
       userData,
+      // A user-data change must actually re-run, so force instance replacement
+      // (the box is stateless — logs live in CloudWatch, the EIP re-associates).
+      userDataCausesReplacement: true,
       blockDevices: [
         {
           deviceName: "/dev/xvda",