docs(openapi): add folder_path, user_id to FileResponse and user_id param to ListFiles

- FileResponse schema: add folder_path (string, nullable), user_id (int64, nullable)

- FileResponse schema: folder_id now nullable instead of omitted

- GET /files: add user_id query parameter for filtering by owner

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

hpc_server_openapi.json

@@ -1381,6 +1381,13 @@
             "required": false,
             "description": "Search files by name",
             "schema": { "type": "string" }
+          },
+          {
+            "name": "user_id",
+            "in": "query",
+            "required": false,
+            "description": "Filter by file owner user ID",
+            "schema": { "type": "integer", "format": "int64" }
           }
         ],
         "responses": {
@@ -2738,7 +2745,18 @@
           "folder_id": {
             "type": "integer",
             "format": "int64",
-            "description": "Parent folder ID",
+            "description": "Parent folder ID (null for root files)",
+            "nullable": true
+          },
+          "folder_path": {
+            "type": "string",
+            "description": "Full folder path (\"/\" for root)",
+            "nullable": true
+          },
+          "user_id": {
+            "type": "integer",
+            "format": "int64",
+            "description": "File owner user ID",
             "nullable": true
           },
           "size": {