feat: track upstream request ID and prevent response header override

When proxying through another new-api instance, the upstream
X-Oneapi-Request-Id was overwriting the local one in client responses.
This adds a new `upstream_request_id` field to the logs table, captures
the upstream ID during relay, and filters it from being copied back to
the client. Frontend gains search/filter and detail display support.

common/constants.go

@@ -179,7 +179,8 @@ var GeminiSafetySetting string
 var CohereSafetySetting string
 
 const (
-	RequestIdKey = "X-Oneapi-Request-Id"
+	RequestIdKey         = "X-Oneapi-Request-Id"
+	UpstreamRequestIdKey = "X-Upstream-Request-Id"
 )
 
 const (