feat(wireless+ports): RF Health panels + physical per-port clients (v0.16.1-beta)

Wireless RF Health (fleet Wireless page + per-AP Radios tab):
- Channel usage map across 2.4/5/6 GHz with co-channel overlap; operating
  channel resolved live from /interface/wifi/monitor on the wifi package
  (auto-channel radios report no frequency in /print).
- AP deployment density: client RSSI distribution on the -90..-30 dBm scale
  with a coverage-gap warning.
- AP radio TX retries histogram (0%->35%+) with band selector, derived from
  per-client CCQ (new wireless_radio_quality InfluxDB series).
- WiFi connectivity success funnel (Association/Auth/DHCP) derived from logs.

Per-port connected clients (Ports tab):
- New card lists clients physically connected to a selected port (host/MAC/
  IP/VLAN/connection time, linked to client pages).
- Distinguishes physical clients from the raw MAC table: uplink/trunk ports
  are auto-detected (topology neighbor, multi-VLAN MAC spread, or high MAC
  count) and show an uplink explainer + "view MAC table" disclosure instead
  of every reachable MAC.
- Added clients.first_seen ("connected since"); set once, preserved across polls.

backend/src/db/migrate.ts

@@ -160,6 +160,9 @@ CREATE TABLE IF NOT EXISTS app_settings (
 ALTER TABLE clients ADD COLUMN IF NOT EXISTS vendor VARCHAR(255);
 ALTER TABLE clients ADD COLUMN IF NOT EXISTS vlan_id INTEGER;
 ALTER TABLE clients ADD COLUMN IF NOT EXISTS custom_name VARCHAR(255);
+-- first_seen: when this client was first discovered ("connected since"); set once, preserved across polls
+ALTER TABLE clients ADD COLUMN IF NOT EXISTS first_seen TIMESTAMPTZ;
+UPDATE clients SET first_seen = last_seen WHERE first_seen IS NULL;
 ALTER TABLE events ADD COLUMN IF NOT EXISTS log_id VARCHAR(20);
 CREATE UNIQUE INDEX IF NOT EXISTS idx_events_device_log_id ON events(device_id, log_id);
 ALTER TABLE topology_links ADD COLUMN IF NOT EXISTS neighbor_mac VARCHAR(17);