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

src/interfaces/api/routes.rs

@@ -11,6 +11,7 @@ use std::sync::Arc;
 use tower_http::{compression::CompressionLayer, trace::TraceLayer};
 use utoipa::OpenApi;
 
+/// Returns the application version from Cargo.toml (compile-time constant)
 async fn get_version() -> AxumJson<serde_json::Value> {
     AxumJson(json!({
         "name": "OxiCloud",