fix(admin,api): honor STORAGE_IMPL when surfacing file_store backend (closes #17294) (#17441)

## Summary

When `STORAGE_IMPL=AWS_S3`, the Admin Service status page keeps showing
MinIO. Three things conspire:

1. `admin/server/config.py::load_configurations` only knows the
   `minio` and `minio_0` config keys. An `s3` block lands on the
   `case _:` branch and logs `Unknown configuration key: s3`
   (issue #17294).
2. `admin/server/services.py::ServiceMgr.get_all_services` filters
   retrieval services by `DOC_ENGINE` but has no equivalent filter
   for `file_store` services by `STORAGE_IMPL`. A stale MinIO block
   is returned regardless of the active backend.
3. The wired health check is hardcoded to `check_minio_alive`, which
   calls `settings.MINIO['host']`. With `STORAGE_IMPL=AWS_S3` that
   block is uninitialized, so the check always times out.


Fixes #17294

api/utils/health_utils.py

@@ -15,6 +15,7 @@
 #
 from datetime import datetime
 import json
+import logging
 import os
 import requests
 from timeit import default_timer as timer
@@ -246,6 +247,21 @@ def check_minio_alive():
         }
 
 
+def check_s3_alive():
+    """
+    Check AWS S3 (or any S3-compatible) liveness via the active
+    storage backend's `.health()` method. Delegates to the generic
+    ``check_storage`` so the same check works for AWS S3, MinIO,
+    R2, and any other S3-compatible endpoint. See #17294.
+    """
+    ok, payload = check_storage()
+    if ok:
+        logging.debug("check_s3_alive: ok, elapsed=%s ms", payload.get("elapsed", "?"))
+        return {"status": "alive", "message": f"Confirm elapsed: {payload.get('elapsed', '?')} ms."}
+    logging.debug("check_s3_alive: failed, error=%s", payload.get("error", "unknown"))
+    return {"status": "timeout", "message": f"error: {payload.get('error', 'unknown')}"}
+
+
 def get_redis_info():
     try:
         return {"status": "alive", "message": REDIS_CONN.info()}