firmware: no-reflash config updates for ESP-01S + UART-OTA groundwork

Updating the buried ESP-01S currently means a USB-UART adapter and a
GPIO0 jumper. Add a path to change its settings without reflashing, and
lay the groundwork for full firmware updates over the existing UART.

set_config (no reflash for settings):
- ESP-01S: add saveConfig() + a set_config command — updates GoPro
  SSID/password/IP and poll interval, persists to LittleFS, acks, and
  re-associates Wi-Fi if creds changed
- XIAO: forward an MQTT set_camera_config down to the ESP-01S over UART
  (hub -> MQTT -> XIAO -> UART -> ESP-01S/LittleFS)

UART-OTA groundwork ("XIAO as flasher"):
- reserve XIAO GPIOs ESP01_RST_PIN=D8, ESP01_PGM_PIN=D10 for driving the
  ESP-01S serial bootloader (not driven yet)
- docs/design/esp01s-uart-ota.md: full design (why Wi-Fi OTA doesn't fit
  the 1MB ESP-01S on the GoPro AP, bootloader entry, ROM flash protocol,
  HTTP-pull delivery, scope)
- hardware/README.md: fix stale ESP32-C3 -> XIAO ESP32-C6 wiring, add the
  two control lines (Notion wiring diagram updated to match)

Both firmwares build clean and are flashed; set_config round-trip needs
the broker to exercise end-to-end.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/design/esp01s-uart-ota.md

@@ -0,0 +1,100 @@
+# ESP-01S firmware updates without a USB-UART adapter
+
+Status: **design / not yet implemented.** Interim mitigations (config-over-UART,
+GPIO reservation) are shipped; see "Shipped now" below.
+
+## Problem
+
+The ESP-01S camera bridge has no native USB. Today it is flashed with an external
+3.3 V USB-UART adapter and a `GPIO0 → GND` jumper held during reset. On an assembled
+field node that is impractical — we want to update it over the network.
+
+## Why not Wi-Fi OTA on the ESP-01S itself
+
+1. **Network topology.** The ESP-01S joins the *GoPro* AP (`10.5.5.1`), not the hub /
+   travel-router network. The hub cannot reach it to push an OTA.
+2. **1 MB flash.** Standard ESP8266 OTA stages a second copy of the sketch alongside the
+   running one. Our sketch is ~333 KB; a 1 MB module has no room for two copies plus FS
+   and reserved areas.
+
+So updates must arrive **through the XIAO**, which is already UART-connected to the
+ESP-01S and sits on the hub network.
+
+## Approach: XIAO ESP32-C6 as the flasher (UART OTA)
+
+The XIAO plays the role the USB-UART adapter plays today, driving the ESP-01S's ROM
+serial bootloader over the existing UART.
+
+### Hardware — two added control lines
+
+| XIAO pin | → ESP-01S | purpose |
+|----------|-----------|---------|
+| `D8` (`ESP01_RST_PIN`) | `RST` | pulse low to reset the ESP-01S |
+| `D10` (`ESP01_PGM_PIN`) | `GPIO0` | hold low across reset → enter bootloader |
+| `D6` (TX) / `D7` (RX) | `RX` / `TX` | existing `Serial1` link (crossed) |
+| GND | GND | common ground |
+
+> **Confirm before committing the PCB/wiring:** verify `D8`/`D10` on the actual XIAO
+> ESP32-C6 variant do **not** map to ESP32-C6 strapping pins (`GPIO8`, `GPIO9`, `GPIO15`)
+> or the USB-JTAG pins. Pins are reserved in firmware (`ESP01_RST_PIN`, `ESP01_PGM_PIN`)
+> but not yet driven.
+
+### Bootloader entry
+
+`GPIO0 = LOW`, pulse `RST` low→high → ESP-01S enters the serial bootloader on the UART.
+After writing: `GPIO0 = HIGH`, pulse `RST` → run the new firmware. Always restore
+`GPIO0 = HIGH` on give-up so the ESP can boot normally.
+
+### Flash protocol
+
+Implement enough of the ESP8266 ROM bootloader / esptool SLIP protocol on the XIAO over
+`Serial1`:
+
+- `SYNC`, then `FLASH_BEGIN` / `FLASH_DATA` (≈1 KB blocks) / `FLASH_END` to write the app
+  at offset `0x0`.
+- Start at 115200 baud; optionally raise after sync.
+- Verify with the ROM `SPI_FLASH_MD5` against the expected MD5.
+
+### Firmware delivery (hub → XIAO)
+
+Greenfield on the Go hub (only a `firmware_version` field exists today). Recommended:
+
+- **HTTP pull.** Hub exposes `GET /firmware/esp01s/<version>.bin` (+ MD5). XIAO is
+  triggered by an MQTT command, e.g.
+  `{"command":"update_esp01s","url":"http://<hub>/firmware/esp01s/0.4.0.bin","md5":"…"}`,
+  fetches the `.bin` in chunks, and streams each chunk straight into a `FLASH_DATA` block.
+- Avoid buffering the whole image in RAM — stream HTTP chunk → flash block → repeat.
+- MQTT chunked transfer is possible but heavier on the broker; prefer HTTP.
+
+### Sequencing / safety
+
+- Pause the UART JSON status/command protocol while flashing (the link is busy with the
+  bootloader protocol).
+- On failure leave the ESP recoverable and retry; report progress/result to the hub over
+  MQTT.
+
+### XIAO self-update (separate, easy)
+
+The XIAO (4 MB flash, on the hub network) can use standard ESP32 OTA (`ArduinoOTA` or
+`httpUpdate`). No gymnastics required. Split: **XIAO = native OTA; ESP-01S = flashed by
+the XIAO over UART.**
+
+## Scope estimate
+
+- **XIAO firmware:** ESP8266 ROM-loader client over `Serial1` + `GPIO0`/`RST` control +
+  HTTP fetch + MQTT trigger. Medium–large.
+- **Hub (Go):** firmware store + HTTP endpoint + MQTT trigger command. Small–medium.
+- **Hardware:** two control wires + confirm non-strapping pins. Small.
+
+## Shipped now (interim)
+
+- **Config-over-UART (`set_config`)** — change GoPro SSID/password/IP and poll interval
+  with no reflash. Hub → `set_camera_config` (MQTT) → XIAO → `set_config` (UART) →
+  ESP-01S persists to LittleFS.
+- **GPIO reservation** — `ESP01_RST_PIN = D8`, `ESP01_PGM_PIN = D10` reserved in the XIAO
+  firmware for the flasher.
+
+## References
+
+- ESP8266 ROM serial bootloader protocol (esptool).
+- Wiring: Notion "XIAO ESP32-C6 Pin-to-Pin Wiring Diagram".