feat(firmware): log boot and reboot reason to event log

Every boot logs EVT_BOOT with esp_reset_reason(); every deliberate
ESP.restart() is preceded by EVT_REBOOT with a reason code. This
gives us a persistent answer to 'why did the device just reboot?'.

firmware/src/main.cpp

@@ -8,6 +8,8 @@
 #include "cv.h"
 #include "ble_scanner.h"
 #include "reporter.h"
+#include "event_log.h"
+#include <esp_system.h>
 
 // LED on GPIO2 (TimerCamera-F built-in LED) — verify against board schematic
 // Factory reset: hold GPIO37 (BOOT button) for 5 seconds
@@ -45,6 +47,7 @@ static void check_factory_reset() {
     uint32_t held = millis();
     while (digitalRead(BUTTON_PIN) == LOW) {
         if (millis() - held >= FACTORY_RESET_HOLD_MS) {
+            event_log_write(EVT_REBOOT, REBOOT_FACTORY_RESET, 0);
             config_clear_wifi();
             ESP.restart();
         }
@@ -140,6 +143,9 @@ void setup() {
     pinMode(BUTTON_PIN, INPUT_PULLUP);
     led_set(true); // on = booting
 
+    event_log_init();
+    event_log_write(EVT_BOOT, (uint16_t)esp_reset_reason(), 0);
+
     if (!config_load(g_cfg)) {
         Serial.println("FATAL: device_id/location_id/hmac_secret not provisioned");
         while (true) { delay(500); led_set(!digitalRead(LED_PIN)); } // fast blink
@@ -148,6 +154,7 @@ void setup() {
     // Connect to WiFi
     if (!config_has_wifi()) {
         provisioning_run();
+        event_log_write(EVT_REBOOT, REBOOT_WIFI_REPROV, 0);
         ESP.restart();
     }
 
@@ -161,6 +168,7 @@ void setup() {
     if (WiFi.status() != WL_CONNECTED) {
         // Saved creds failed — re-provision
         provisioning_run();
+        event_log_write(EVT_REBOOT, REBOOT_WIFI_REPROV, 0);
         ESP.restart();
     }
 
@@ -183,7 +191,11 @@ void setup() {
     // OTA update support
     ArduinoOTA.setHostname(g_cfg.device_id.c_str());
     ArduinoOTA.onStart([]() { ble_scanner_pause(); });
-    ArduinoOTA.onEnd([]()   { ble_scanner_resume(); ESP.restart(); });
+    ArduinoOTA.onEnd([]()   {
+        ble_scanner_resume();
+        event_log_write(EVT_REBOOT, REBOOT_OTA, 0);
+        ESP.restart();
+    });
     ArduinoOTA.onError([](ota_error_t e) { ble_scanner_resume(); });
     ArduinoOTA.begin();