chore: remove generated openapi.json from tracking, add test and docs

- Remove resources/gen/openapi.json from git (served dynamically at /api/openapi.json)
- Add resources/gen/ to .gitignore
- Add OpenAPI spec validation test (paths, schemas, serialization)
- Restore removed doc-comment on get_version
- Fix cargo fmt violation in mod.rs import
- Update CLAUDE.md: test count (~208), generate-openapi command, justfile reference

.gitignore

@@ -78,5 +78,8 @@ storage/
 *.swo
 nohup.out
 
+# Generated files (OpenAPI spec, etc.)
+resources/gen/
+
 # Helm chart dependencies
 charts/*/charts/*