fix(api/file): prevent path traversal in file API

Use os.OpenRoot to restrict file access to the application root,
preventing directory traversal attacks through the file download endpoint.

Also add test to verify path traversal attempts are blocked.

internal/api/v1/file/get.go

@@ -1,6 +1,7 @@
 package fileapi
 
 import (
+	"io"
 	"net/http"
 	"os"
 	"path"
@@ -44,7 +45,14 @@ func Get(c *gin.Context) {
 		return
 	}
 
-	content, err := os.ReadFile(request.FileType.GetPath(request.Filename))
+	f, err := os.OpenInRoot(".", request.FileType.GetPath(request.Filename))
+	if err != nil {
+		c.Error(apitypes.InternalServerError(err, "failed to open root"))
+		return
+	}
+	defer f.Close()
+
+	content, err := io.ReadAll(f)
 	if err != nil {
 		c.Error(apitypes.InternalServerError(err, "failed to read file"))
 		return