* feat(docker): add jemalloc to reduce memory fragmentation
Install libjemalloc2 in the base image and preload it via LD_PRELOAD in
docker-entrypoint when available. Reduces RSS growth from glibc's default
allocator fragmentation under Rails workloads.
* feat(docker): add DISABLE_JEMALLOC env var + preserve existing LD_PRELOAD
* feat(docker): add jemalloc status logging to entrypoint
* refactor(docker): simplify jemalloc logging to warn-only when disabled/missing
