S3 Lifecycle management (expiration/transition + sweeper) #22

PutBucketLifecycleConfiguration expiration/transition rules that actually delete or
tier objects after N days, surfaced via x-amz-expiration, are unimplemented. (The
Expires caching header is implemented and is unrelated — it never deletes.) This
is its own subsystem: a per-bucket rule store + a background sweeper driving the
reference index.

Refs: docs/architecture.md §12 "object lifecycle (not implemented)"; code:
bucket/manifest.go (Expires).

Part of #17