Step 2b: firmware upload (/api/ota) with image checks and rollback self-test

- aes67_ota: GET /api/ota (version, project, build_date, idf, running,
  previous, previous_version, pending_verify, can_rollback), POST
  /api/ota streamed into the inactive slot (4 KiB chunks), POST
  /api/ota/confirm and /api/ota/rollback.
- Rejected before any flash write: bad magic, wrong chip, wrong
  project_name, chip revision outside the image's min/max range.
  esp_ota_end verifies the whole image.
- Self-test on a pending image: IP address plus HTTP 200 from our own
  /api/status within 60 s marks it valid; otherwise mark invalid and
  reboot into the previous slot. A crash before that is rolled back by
  the bootloader.
- CONFIG_AES67_OTA_SELFTEST_FORCE_FAIL (test builds only) forces a
  failed self-test.
- aes67_web: web_reboot_later() shared by /api/reboot and OTA.

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

components/aes67_ota/Kconfig

@@ -0,0 +1,11 @@
+menu "AES67 OTA"
+
+    config AES67_OTA_SELFTEST_FORCE_FAIL
+        bool "Force the OTA self-test to fail (rollback test only)"
+        default n
+        help
+            Test builds only: the self-test always fails, so a freshly installed
+            image rolls back. Build with sdkconfig.selftest_fail in a separate
+            build directory; never enable in the normal build.
+
+endmenu