fix: persist upload original filename

docs/sql/IM-文件上传原始文件名字段SQL.sql

@@ -0,0 +1,5 @@
+-- Persist client-provided upload filename for downstream business modules.
+-- Apply before deploying im_server changes which read/write im_fileuploads.original_name.
+
+ALTER TABLE im_fileuploads
+  ADD COLUMN original_name VARCHAR(255) NOT NULL DEFAULT '' AFTER location;