feat: say when the firmware is flashed over USB

Builds with one app slot (C6, debug, and the C5 to come) have nowhere
for /updateFirmware to write. It answered "Error updating firmware: "
and rebooted.

- ConfigSettings::otaSlot, from esp_ota_get_next_update_partition().
- The header sends otaSlot, default 1, so only those builds send it.
- /updateFirmware refuses with 400 and a reason, and doesn't stop the
  radio or reboot.
- The web UI's Update Firmware says to flash the onboard image over USB,
  with a backup first. Firmware before the field leaves it undefined,
  which keeps the uploader.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ConfigSettings.cpp

@@ -7,6 +7,7 @@
 #include "Utils.h"
 #include "Secrets.h"
 #include "esp_chip_info.h"
+#include "esp_ota_ops.h"
 #include "esp_log.h"
 
 static const char *TAG = "Config";
@@ -144,6 +145,7 @@ bool ConfigSettings::begin() {
       break;
   }
   ESP_LOGD(TAG, "Chip Model ESP32-%s", this->chipModel);
+  this->otaSlot = esp_ota_get_next_update_partition(nullptr) != nullptr;
   this->fwVersion.parse(FW_VERSION);
   uint64_t mac = ESP.getEfuseMac();
   for(int i=0; i<17; i=i+8) {

src/ConfigSettings.h

@@ -163,6 +163,9 @@ class ConfigSettings: BaseSettings {
     char serverId[10] = "";
     char hostname[32] = "ESPSomfyRTS";
     char chipModel[10] = "ESP32";
+    // Whether the partition table has a second app slot for /updateFirmware
+    // to write.  Builds with one 3 MB slot (C5, C6, debug) are flashed over USB.
+    bool otaSlot = true;
     conn_types_t connType = conn_types_t::unset;
     appver_t fwVersion;
     appver_t appVersion;

src/ProtoSchema.h

@@ -169,6 +169,7 @@
   FIELD(hostname, "hn", 's', 0) \
   FIELD(model, "md", 's', 0) \
   FIELD(chipModel, "cp", 's', 0) \
+  FIELD(otaSlot, "os", 'b', 1) \
   FIELD(connType, "ct", 's', 0) \
   /* memory, wifi and ethernet */ \
   FIELD(freeHeap, "fr", 'i', 0) \

src/entryPoints/HttpRoutes.cpp

@@ -164,6 +164,11 @@ void registerHttpRoutes(AsyncWebServer &server) {
   // updateFirmware - file upload
   server.on(AsyncURIMatcher::exact("/updateFirmware"), HTTP_POST,
     [](AsyncWebServerRequest *request) {
+      // Nothing was written or stopped, so there is nothing to reboot for.
+      if(!settings.otaSlot) {
+        request->send(400, _encoding_json, F("{\"status\":\"ERROR\",\"desc\":\"This device has no OTA slot, so its firmware is flashed over USB.\"}"));
+        return;
+      }
       if(Update.hasError())
         request->send(500, _encoding_json, "{\"status\":\"ERROR\",\"desc\":\"Error updating firmware: \"}");
       else
@@ -172,7 +177,7 @@ void registerHttpRoutes(AsyncWebServer &server) {
       rebootDelay.rebootTime = millis() + 500;
     },
     [](AsyncWebServerRequest *request, String filename, size_t index, uint8_t *data, size_t len, bool final) {
-      if(!uploadAllowed(request, index)) return;
+      if(!uploadAllowed(request, index) || !settings.otaSlot) return;
       if(index == 0) {
         webServer.uploadSuccess = false;
         ESP_LOGI(TAG, "Update: %s", filename.c_str());

src/outputs/ProtoSocket.cpp

@@ -77,6 +77,7 @@ static void writeHeader(ProtoWriter &w) {
   w.add(proto_field_t::hostname, settings.hostname);
   w.add(proto_field_t::model, "ESPSomfyRTS");
   w.add(proto_field_t::chipModel, settings.chipModel);
+  w.add(proto_field_t::otaSlot, settings.otaSlot);
   w.add(proto_field_t::fw, settings.fwVersion.name);
   w.add(proto_field_t::connType, net.connTypeName());
 }