feat: NetFlow/IPFIX traffic analytics — per-client usage, app breakdown, built-in collector (v0.13.0-beta)

Adds Meraki-style traffic analytics powered by MikroTik Traffic Flow:

Backend
- Built-in NetFlow v9 + IPFIX collector (UDP 2055) with hand-rolled decoder,
  template caching, enterprise-field and IPv6 support
- Flow attribution: client IP → MAC via the clients table, upload/download
  direction, NAT-duplicate records dropped
- App classification by protocol/port (~25 categories: HTTPS, QUIC, DNS,
  SSH, Email, WireGuard, ...)
- Deduplication: per-client best-exporter-per-window so flows crossing two
  managed routers are never double-counted
- NAT-tolerant ingest: flows from sources matching no managed device (NAT /
  Tailscale subnet routers in path) are accepted as per-source unidentified
  exporters instead of silently dropped (netflow_accept_unknown setting)
- Storage: InfluxDB client_traffic time-series (bounded cardinality via
  top-N fold) + Postgres client_traffic_daily rollups; daily retention purge
- One-toggle device config push via RouterOS API (/ip traffic-flow + target)
- New /api/traffic/* analytics endpoints + /api/network-services/netflow
- docker-compose: NETFLOW_PORT (default 2055/udp) published on the backend

Frontend
- New Traffic page: fleet up/down chart, top talkers with client
  click-through, app-category donut, 1h–30d ranges
- New NetFlow config page: collector settings, live ingest stats,
  per-device export toggles with health badges, NAT-source banner
- Client detail: App Traffic card (auto-hides without data)
- Clients page: sortable "Data (today)" column
- Settings: NetFlow retention controls

Includes 17 unit tests (decoder fixtures, dedup, attribution, classifier);
verified end-to-end against live RouterOS devices.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/pages/SettingsPage.tsx

@@ -667,6 +667,8 @@ export default function SettingsPage() {
               {[
                 { key: 'retention_events_days', label: 'Events retention', desc: 'Auto-delete event log entries older than this many days' },
                 { key: 'retention_clients_days', label: 'Client retention', desc: 'Auto-delete inactive client records not seen within this many days' },
+                { key: 'netflow_retention_days', label: 'Traffic detail retention', desc: 'Auto-delete per-client NetFlow time-series data older than this many days' },
+                { key: 'netflow_daily_retention_days', label: 'Traffic daily rollup retention', desc: 'Auto-delete per-client daily traffic totals older than this many days' },
               ].map(({ key, label, desc }) => (
                 <div key={key} className="flex items-center justify-between gap-4">
                   <div>