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>

firmware/src/esp32-mqtt-bridge.cpp

@@ -113,6 +113,13 @@ int batteryPct(int raw) {
 #define UART_RX_PIN  D7
 #define UART_TX_PIN  D6
 
+// Reserved for future ESP-01S UART OTA ("XIAO as flasher"): two control
+// lines let the XIAO drive the ESP-01S into its serial bootloader and
+// reflash it over Serial1 — no USB-UART adapter or GPIO0 jumper needed.
+// Not driven yet; see docs/design/esp01s-uart-ota.md.
+#define ESP01_RST_PIN D8    // → ESP-01S RST   (pulse low to reset)
+#define ESP01_PGM_PIN D10   // → ESP-01S GPIO0 (low at reset = bootloader)
+
 // ────────────────────────────────────────────
 // RGB STAT LED — D0/D1/D2 (red/green/blue) via 220Ω each
 // ────────────────────────────────────────────
@@ -263,6 +270,20 @@ void mqttCallback(char* topic, byte* payload, unsigned int len) {
     saveConfig();
     Serial.printf("[BAT] Calibration set: raw_min=%d raw_max=%d\n",
                   cfg.bat_raw_min, cfg.bat_raw_max);
+  } else if (cmd == "set_camera_config") {
+    // Forward camera-bridge config to the ESP-01S over UART so the
+    // GoPro creds / poll rate can change without reflashing it.
+    JsonDocument out;
+    out["type"]    = "cmd";
+    out["command"] = "set_config";
+    if (!doc["camera_ssid"].isNull())       out["camera_ssid"]       = doc["camera_ssid"];
+    if (!doc["camera_password"].isNull())   out["camera_password"]   = doc["camera_password"];
+    if (!doc["camera_ip"].isNull())         out["camera_ip"]         = doc["camera_ip"];
+    if (!doc["poll_interval_sec"].isNull()) out["poll_interval_sec"] = doc["poll_interval_sec"];
+    String line; serializeJson(out, line);
+    UART_ESP8266.println(line);
+    UART_ESP8266.flush();
+    Serial.println("[MQTT] Forwarded set_config → ESP-01S");
   } else if (cmd == "registered") {
     String id = doc["camera_id"] | "";
     if (id.length() > 0 && id != cfg.camera_id) {