fix(mnemosyne): use STORAGES config for S3 health check

Update `_check_s3` to read S3 settings from the `STORAGES` dict instead of
deprecated top-level `AWS_*` settings. Skip the check when local storage
is enabled and return an error early if no bucket is configured.

mnemosyne/mnemosyne/views.py

@@ -58,17 +58,25 @@ def _check_s3() -> str | None:
     import botocore.exceptions
     from django.conf import settings
 
+    if getattr(settings, "USE_LOCAL_STORAGE", False):
+        return None
+
+    opts = settings.STORAGES.get("default", {}).get("OPTIONS", {})
+    bucket = opts.get("bucket_name", "")
+    if not bucket:
+        return "No S3 bucket configured"
+
     try:
         client = boto3.client(
             "s3",
-            endpoint_url=settings.AWS_S3_ENDPOINT_URL or None,
-            aws_access_key_id=settings.AWS_ACCESS_KEY_ID or None,
-            aws_secret_access_key=settings.AWS_SECRET_ACCESS_KEY or None,
-            region_name=settings.AWS_S3_REGION_NAME or None,
-            use_ssl=getattr(settings, "AWS_S3_USE_SSL", True),
-            verify=getattr(settings, "AWS_S3_VERIFY", True),
+            endpoint_url=opts.get("endpoint_url") or None,
+            aws_access_key_id=opts.get("access_key") or None,
+            aws_secret_access_key=opts.get("secret_key") or None,
+            region_name=opts.get("region_name") or None,
+            use_ssl=opts.get("use_ssl", True),
+            verify=opts.get("verify", True),
         )
-        client.head_bucket(Bucket=settings.AWS_STORAGE_BUCKET_NAME)
+        client.head_bucket(Bucket=bucket)
         return None
     except botocore.exceptions.ClientError as e:
         return f"HTTP {e.response['Error']['Code']}"