Optimize bucket listing for 10K-100K objects

- Shallow listing: read per-directory _index.json once for eTags instead
  of N serial .meta.json reads. Validate prefix for path traversal and
  verify normalized target stays within bucket root.
- Recursive listing: cache full per-directory index during the walk so
  each _index.json is parsed at most once per call.
- Per-bucket listing cache with 5s TTL and per-bucket rebuild mutex.
  Invalidated on put/delete/copy/metadata/tags/multipart-complete.
  Pagination uses partition_point for O(log n) start lookup.
- UI stream endpoint now actually streams via mpsc + Body::from_stream
  instead of buffering into a Vec<String>. Cancels producer on client
  disconnect.
- UI JSON endpoint honors delimiter=/ and returns common_prefixes.
- run_blocking wrapper dispatches sync filesystem work via
  block_in_place on multi-threaded runtimes, falls back to inline on
  current-thread runtimes (unit tests).

Cargo.toml

@@ -43,6 +43,7 @@ thiserror = "2"
 chrono = { version = "0.4", features = ["serde"] }
 base64 = "0.22"
 tokio-util = { version = "0.7", features = ["io"] }
+tokio-stream = "0.1"
 futures = "0.3"
 dashmap = "6"
 crc32fast = "1"