chore: ignore the installer's log.json

`cmd/installer/wizard/logger` opens log.json from its init(), so importing the
package is enough to create one in the process's working directory. Running
`go test ./cmd/installer/...` therefore drops a log.json into every package
directory it runs from — four of them were sitting untracked in the tree,
one step away from an accidental `git add -A`.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -8,6 +8,7 @@
 backend/tmp
 backend/build
 backend/vendor
+backend/cmd/installer/**/log.json
 
 frontend/coverage
 frontend/dist