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

hardware/README.md

@@ -23,11 +23,13 @@ Each camera node is a self-contained unit clipped onto a GoPro Hero 3. It provid
 │  │        Screen           │    │
 │  └─────────────────────────┘    │
 │                    ┌──────────┐ │
-│    3D Sleeve ─────→│ ESP32    │ │ ← clips onto back/bottom
+│    3D Sleeve ─────→│ ESP8266  │ │ ← Camera bridge (GoPro Wi-Fi)
 │                    │ D1 Mini  │ │
-│                    └──────────┘ │
-│                    ┌──────────┐ │
-│                    │ LiPo     │ │ ← slides under GoPro
+│                    ├──────────┤ │
+│                    │ ESP32    │ │ ← MQTT bridge (travel router)
+│                    │ Dev      │ │
+│                    ├──────────┤ │
+│                    │ LiPo     │ │ ← Shared power
 │                    │ 1000mAh  │ │
 │                    └──────────┘ │
 └─────────────────────────────────┘
@@ -38,16 +40,18 @@ Each camera node is a self-contained unit clipped onto a GoPro Hero 3. It provid
 | Item | Qty | Cost | Notes |
 |------|-----|------|-------|
 | GoPro Hero 3 Black/Silver | 1 | Already owned | Target camera |
-| ESP32 D1 Mini | 1 | ~$4 | Or NodeMCU-32S (~$5) |
+| ESP32 Dev Board | 1 | ~$5 | MQTT bridge — talks to hub |
+| ESP8266 D1 Mini | 1 | ~$3 | Camera bridge — talks to GoPro |
 | LiPo 3.7V 1000mAh | 1 | ~$8 | 50x34x8mm typical |
-| 5V/3A buck converter | 1 | ~$2 | LiPo → GoPro USB |
-| 3.3V buck converter | 1 | ~$1 | LiPo → ESP32 VIN |
+| 3.3V buck converter | 1 | ~$1 | LiPo → both boards (shared VIN) |
+| 5V/3A buck converter | 1 | ~$2 | LiPo → GoPro USB (power only) |
 | JST-XH 2-pin connectors | 2 | ~$1 | Battery quick-disconnect |
 | Micro-USB right-angle cable | 1 | ~$2 | Buck → GoPro |
+| Jumper wires (female-female) | 4 | ~$0.50 | UART + GND between boards |
 | Velcro strap (20cm) | 1 | ~$0.50 | Secure to GoPro |
-| PETG filament | ~30g | ~$0.60 | 3D printed case |
+| PETG filament | ~35g | ~$0.70 | 3D printed case |
 
-**Total per node:** ~$20
+**Total per node:** ~$24
 
 ## 3D Printed Case
 
@@ -88,17 +92,17 @@ Slides under the GoPro. Contains:
 LiPo 3.7V
   ├── JST-XH connector
   │
-  ├──→ 5V/3A Buck Converter → Micro-USB right-angle → GoPro USB port
-  │    (power only — no data over USB)
+  ├──→ 3.3V Buck Converter → ESP8266 VIN + GND
+  │                        → ESP32 VIN + GND
+  │                        (both boards share the same 3.3V rail)
   │
-  └──→ 3.3V Buck Converter → ESP32 VIN + GND
-       (or ESP32 D1 Mini has built-in regulator — connect directly to 5V pin)
-```
+  └──→ 5V/3A Buck Converter → Micro-USB right-angle → GoPro USB port
+       (power only — no data over USB)
 
-**Note:** ESP32 D1 Mini has an onboard 3.3V regulator. You can feed it 5V directly to the 5V pin if using a single 5V buck converter. This simplifies wiring:
-```
-LiPo → 5V Buck → ├── ESP32 5V pin
-                  └── GoPro USB port
+UART (ESP8266 ↔ ESP32):
+  ESP8266 TX (GPIO1)  ──→ ESP32 RX (GPIO16)
+  ESP8266 RX (GPIO3)  ←── ESP32 TX (GPIO17)
+  ESP8266 GND ─────────── ESP32 GND
 ```
 
 ## Wi-Fi Topology (No Cables for Camera Control)