feat: add OTA firmware update from URL

Wraps esp_https_ota for over-the-air updates via HTTPS.
Dual OTA partitions enable safe rollback.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

main/ota/ota_manager.h

@@ -0,0 +1,12 @@
+#pragma once
+
+#include "esp_err.h"
+
+/**
+ * Perform OTA firmware update from a URL.
+ * Downloads the firmware binary and applies it. Reboots on success.
+ *
+ * @param url  HTTPS URL to the firmware .bin file
+ * @return ESP_OK on success (device will reboot), error code otherwise
+ */
+esp_err_t ota_update_from_url(const char *url);