Feature detail batch (lazy-loaded)

Fan-out variant of /features/{id}/details for callers that need to resolve many features at once. POST is used so the id list isn't constrained by URL length.

Per-id caching is identical to the single endpoint (Redis-backed keyed on the id), so a batch that's a superset of previously requested ids costs nothing extra for the overlap. Partial success is preserved: a single failed upstream fetch sets cache: "error" with an error string on that entry without failing the whole request — or cache: "error-cached" when a recent failure was replayed from the negative cache instead of calling the source again. Missing ids land in the response with not_found: true.

Bounded by the plan's max_batch_size.