feat: implement OTA updates over HTTP(S), initiated over MQTT

components/ota_manager/include/ota_manager.h

@@ -0,0 +1,12 @@
+#pragma once
+
+#include "esp_err.h"
+
+/**
+ * Begin an OTA update in a background task.
+ * Applies a MAC-derived jitter delay before downloading so a fleet-wide
+ * trigger staggers reboots instead of dropping all coverage at once.
+ * Reports status via mqtt_publisher_send_ota_status().
+ * On success the device reboots; on failure it stays on the current firmware.
+ */
+esp_err_t ota_manager_start(const char *url, const char *version);