OTA follow-up: align with ESP-IDF best practices (rollback + evaluate pull OTA)

[21cncstudio]

Summary

Current web OTA is functionally working and already uses a safe dual-slot layout (otadata + ota_0 + ota_1), but it does not yet fully match the OTA recommendations from Espressif.

What we already have

• Safe app OTA partition layout
• OTA writes to the inactive app slot
• Slot size checks and oversized image rejection
• Improved web OTA state handling, timeout policy, and concurrent upload rejection

What is still missing

• App rollback on first boot validation
• Fast self-test after first boot of a new OTA image
• Optional anti-rollback / secure_version checks
• If we ever move to internet OTA: HTTPS OTA with server verification and optional image signature verification
• Evaluation of device-pull OTA (esp_https_ota) with resume support for poor/unreliable networks

Why this matters

• Espressif recommends rollback with first-boot validation so a bad OTA image can automatically fall back to the previous working firmware.
• The current browser-upload OTA path is still dependent on the client/browser/network upload path.
• A future device-pull OTA flow may be more reliable on problematic networks, especially with OTA resumption support.

References

• ESP-IDF OTA guide:
  https://docs.espressif.com/projects/esp-idf/en/stable/esp32/api-reference/system/ota.html
• ESP HTTPS OTA guide:
  https://docs.espressif.com/projects/esp-idf/en/stable/esp32c6/api-reference/system/esp_https_ota.html

Proposed follow-up

1. Add rollback support (CONFIG_BOOTLOADER_APP_ROLLBACK_ENABLE) and first-boot self-test flow.
2. Confirm new firmware with esp_ota_mark_app_valid_cancel_rollback() after successful diagnostics.
3. On failure, roll back with esp_ota_mark_app_invalid_rollback_and_reboot().
4. Separately evaluate a future pull OTA path using esp_https_ota.
5. If pull OTA is adopted, consider HTTPS certificate validation, optional signature verification, and OTA resumption.

Not urgent. This is a follow-up improvement task for later.