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) {

firmware/src/esp8266-camera-bridge.cpp

@@ -16,17 +16,23 @@
  *   ESP32 → ESP8266: {"type":"cmd","command":"start_recording"}\n
  *
  * Hardware:
- *   - ESP8266 D1 Mini (or NodeMCU)
- *   - UART TX → ESP32 RX (GPIO 16)
- *   - UART RX → ESP32 TX (GPIO 16)
+ *   - ESP-01S (ESP8266, 1MB flash) on its own 3.3V buck
+ *   - UART is the hardware Serial (GPIO1 TX / GPIO3 RX), crossed:
+ *       ESP-01S TX (GPIO1) → XIAO D7 (RX)
+ *       ESP-01S RX (GPIO3) ← XIAO D6 (TX)
  *   - Shared GND between boards
- *   - LiPo → 3.3V buck → VIN on both boards
+ *   - Flash with a 3.3V USB-UART adapter, GPIO0 → GND on power-up
+ *
+ * Note: the JSON protocol shares the same UART as the boot-ROM/debug
+ * output, so the ESP32 also sees boot chatter and ignores it as
+ * non-JSON. There is no spare pin for a status LED on the ESP-01S
+ * (GPIO1 is the UART TX) — status is shown on the XIAO panel instead.
  */
 
 #include <Arduino.h>
 #include <ESP8266WiFi.h>
 #include <WiFiClient.h>
-#include <HTTPClient.h>
+#include <ESP8266HTTPClient.h>
 #include <ArduinoJson.h>
 #include <LittleFS.h>
 
@@ -58,6 +64,22 @@ bool loadConfig() {
   return true;
 }
 
+// Persist current config to LittleFS. Lets the hub update camera
+// credentials/poll rate over UART without reflashing the ESP-01S.
+bool saveConfig() {
+  if (!LittleFS.begin()) { Serial.println("[CFG] LittleFS mount failed"); return false; }
+  File f = LittleFS.open("/config.json", "w");
+  if (!f) { Serial.println("[CFG] open for write failed"); return false; }
+  JsonDocument doc;
+  doc["camera_ssid"]       = cfg.camera_ssid;
+  doc["camera_password"]   = cfg.camera_password;
+  doc["camera_ip"]         = cfg.camera_ip;
+  doc["poll_interval_sec"] = cfg.poll_interval_sec;
+  serializeJson(doc, f);
+  f.close();
+  return true;
+}
+
 // ────────────────────────────────────────────
 // Camera HTTP Client (GoPro Hero 3)
 // ────────────────────────────────────────────
@@ -173,6 +195,21 @@ void handleCommand(const JsonDocument& doc) {
     pong["type"] = "pong";
     pong["uptime_ms"] = millis();
     sendToESP32(pong);
+  } else if (cmd == "set_config") {
+    // No-reflash config update from the hub (via the XIAO over UART).
+    // Only provided fields change; the rest keep their current value.
+    String oldSsid = cfg.camera_ssid, oldPw = cfg.camera_password;
+    cfg.camera_ssid       = doc["camera_ssid"]     | cfg.camera_ssid;
+    cfg.camera_password   = doc["camera_password"] | cfg.camera_password;
+    cfg.camera_ip         = doc["camera_ip"]       | cfg.camera_ip;
+    cfg.poll_interval_sec = doc["poll_interval_sec"] | cfg.poll_interval_sec;
+    saveConfig();
+    sendAck("set_config");
+    // Re-associate if the camera Wi-Fi credentials changed.
+    if (cfg.camera_ssid != oldSsid || cfg.camera_password != oldPw) {
+      WiFi.disconnect();
+      WiFi.begin(cfg.camera_ssid.c_str(), cfg.camera_password.c_str());
+    }
   } else {
     sendError("Unknown command: " + cmd);
   }
@@ -197,26 +234,16 @@ bool readLine(String& line) {
   return false;
 }
 
-// ────────────────────────────────────────────
-// LED
-// ────────────────────────────────────────────
-
-const int LED = LED_BUILTIN; // active-low on ESP8266 D1 Mini
-
-void ledOn()  { digitalWrite(LED, LOW); }
-void ledOff() { digitalWrite(LED, HIGH); }
-
 // ────────────────────────────────────────────
 // Setup
 // ────────────────────────────────────────────
+// No status LED: GPIO1 is the UART TX to the XIAO and GPIO3 is RX,
+// leaving no free pin on the ESP-01S. Status lives on the XIAO panel.
 
 void setup() {
   Serial.begin(115200);
   delay(500);
-  Serial.println("\n[BRIDGE] ESP8266 Camera Bridge v1.0");
-
-  pinMode(LED, OUTPUT);
-  ledOff();
+  Serial.println("\n[BRIDGE] ESP-01S Camera Bridge v1.0");
 
   loadConfig();
 
@@ -232,7 +259,6 @@ void setup() {
 
   if (WiFi.status() == WL_CONNECTED) {
     Serial.printf("\n[WIFI] Connected. IP: %s\n", WiFi.localIP().toString().c_str());
-    ledOn(); // Solid = connected
   } else {
     Serial.println("\n[WIFI] FAILED — will retry in loop");
   }
@@ -246,7 +272,6 @@ void loop() {
   unsigned long now = millis();
   static unsigned long lastPoll = 0;
   static unsigned long lastWiFiRetry = 0;
-  static bool cameraOnline = false;
 
   // ── Wi-Fi reconnection ──
   if (WiFi.status() != WL_CONNECTED && now - lastWiFiRetry > 10000) {
@@ -261,15 +286,6 @@ void loop() {
 
     if (WiFi.status() == WL_CONNECTED) {
       CamStatus s = fetchStatus();
-      
-      if (s.valid && !cameraOnline) {
-        cameraOnline = true;
-        ledOn();
-      } else if (!s.valid && cameraOnline) {
-        cameraOnline = false;
-        ledOff();
-      }
-      
       sendStatus(s);
     } else {
       // Offline — send empty status so ESP32 knows we're alive but camera is down
@@ -291,13 +307,4 @@ void loop() {
       // Ignore other message types — they're for the ESP32
     }
   }
-
-  // ── LED blink when offline ──
-  if (!cameraOnline) {
-    static unsigned long lastBlink = 0;
-    if (now - lastBlink > 500) {
-      lastBlink = now;
-      digitalWrite(LED, !digitalRead(LED));
-    }
-  }
 }