feat: dual-board architecture — ESP8266 camera bridge + ESP32 MQTT bridge

Complete rewrite of firmware into two dedicated boards per camera node:

ESP8266 (Camera Bridge):
- Connects ONLY to GoPro AP — polls status, sends over UART
- Zero network switching, zero MQTT
- HTTP GET /bacpac/SH for status, start/stop
- JSON-per-line UART protocol to ESP32

ESP32 (MQTT Bridge):
- Connects ONLY to travel router — MQTT to Pi hub
- Reads status from ESP8266 over UART2 (RX16/TX17)
- Auto-registration, heartbeat, command forwarding
- Zero camera communication

UART Protocol: JSON-per-line at 115200 8N1
  ESP8266→ESP32: status/ack/pong/error
  ESP32→ESP8266: cmd (start_recording/stop_recording/ping)

Hardware updates:
- BOM now includes both boards (~4/node)
- 3D case has stacked dual-board compartment
- UART wire channel between board recesses
- Shared 3.3V power rail for both boards

firmware/data/esp32-config.json

@@ -0,0 +1,8 @@
+{
+  "wifi_ssid": "RemoteRig",
+  "wifi_password": "",
+  "mqtt_broker": "192.168.4.10",
+  "mqtt_port": 1883,
+  "camera_id": "",
+  "heartbeat_interval_sec": 60
+}