feat(mqtt): report Wi-Fi signal and LAN throughput telemetry

[gnacho]

The telemetry added in #401 exposed cumulative per-client byte counters, which read
zero on most devices (no per-station accounting), and had no Wi-Fi health figure.

Changes:

• Report the weakest and the average signal strength of the Wi-Fi clients.
• Report LAN throughput in Mbit/s, plus the cumulative totals, taken from the LAN
  bridge counters: the same source the panel traffic graph already uses. The bridge
  exists on every device, including the APs that have no WAN at all.

Verification:

• go test ./... and go vet pass.
• Deployed on rt3 (arm64) and rt-lab (mipsle): Home Assistant discovery now publishes
  23 entities per router (4 new: weakest signal, average signal, LAN traffic in/out).
• Live on rt3: rx_bytes 6,037,105,154 and a rate of 0.08 Mbit/s over a 61 s window
  (+593,885 B), which matches the counters. The signal figures read zero until a
  Wi-Fi client is connected; the aggregation is covered by tests (-50/-80 dBm give
  min -80 and average -65).

Closes #403