feat: add /api/v1/language endpoint for Go/Python runtime detection (#16952)

Both backends serve GET /api/v1/language. Frontend calls it once and
caches. By this way, front end can know the backend is go or python and
thus can determine which part of logic to load.

---------

Co-authored-by: Claude <noreply@anthropic.com>

internal/handler/system.go

@@ -260,3 +260,9 @@ func (h *SystemHandler) ListEnvironments(c *gin.Context) {
 
 	common.SuccessWithData(c, environments, "SUCCESS")
 }
+
+// Language returns the backend runtime language so the front end can
+// choose the appropriate code path (Go vs Python).
+func (h *SystemHandler) Language(c *gin.Context) {
+	common.SuccessWithData(c, map[string]string{"language": "go"}, "success")
+}