Refactor system API (#13958)

### What problem does this PR solve?

- ping
- token
- log level

### Type of change

- [x] Refactoring


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Refactor**
* System endpoints consolidated under /api/v1/system: ping, health
check, and token management moved to the centralized API surface.
* Token management unified at /api/v1/system/tokens with
list/create/delete behavior.

* **Documentation**
  * API reference updated to reflect the new /api/v1/system paths.

* **Tests**
* Client fixtures and test utilities updated to use
/api/v1/system/tokens; one unit test for health/oceanbase status
removed.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

api/utils/health_utils.py

@@ -290,7 +290,7 @@ def get_redis_info():
 def check_ragflow_server_alive():
     start_time = timer()
     try:
-        url = f'http://{settings.HOST_IP}:{settings.HOST_PORT}/v1/system/ping'
+        url = f'http://{settings.HOST_IP}:{settings.HOST_PORT}/api/v1/system/ping'
         if '0.0.0.0' in url:
             url = url.replace('0.0.0.0', '127.0.0.1')
         response = requests.get(url)