feat(scanner): expose prefix-level bucket usage via admin API (HS-08) (#6171)

feat(scanner): expose prefix-level bucket usage via admin API

The scanner's per-bucket, per-set usage caches already hold a path-keyed
prefix tree, but dui() flattened it only to bucket names — consoles and
operators had no way to ask "what does this prefix hold" without an S3
listing sweep (rustfs/backlog#1872, MinIO loadPrefixUsageFromBackend
parity).

Add:

- data-usage: prefix_usage_in_cache — a shared aggregation over the
  entry map (arbitrary prefix, full counters, one-level sub-prefix
  breakdown with names recovered from the literal-path cache keys),
  hardened like the scanner's checked flatten: cycles, dangling child
  links, over-deep trees, and overflowing counters yield None rather
  than unbounded recursion or wrapped totals.
- ecstore: ECStore::all_set_disks — iterate every erasure set so a
  query can read each set's own cache copy; the hash-routed store path
  would always land on one set.
- scanner: bucket_prefix_usage — per-set loads (5s budget each, a slow
  set degrades to not-reporting instead of stalling the caller),
  merged across sets with partial/compacted/truncated flags, served
  from a bounded 30s cache (128 entries, hard-capped) that bucket
  writes invalidate through the dirty-usage hook.
- admin: GET /rustfs/admin/v3/usage/{bucket}?prefix=&max-entries=
  behind the same any-of gate as datausageinfo (DataUsageInfoAdminAction
  OR ListBucketAction), rejecting unknown query parameters and
  clamping max-entries to 1..=10000. Route registered in the policy
  table (deferred MultipleActions, matching datausageinfo) and the
  route matrix test.

Closes rustfs/backlog#1872.

Co-authored-by: heihutu <heihutu@gmail.com>

rustfs/src/admin/handlers/system.rs

@@ -1158,10 +1158,10 @@ impl Operation for RuntimeCapabilitiesHandler {
     }
 }
 
-/// Authorization gate for GET datausageinfo: any-of the dedicated admin action
-/// OR the bucket listing action. Pinned by a unit test so the gate cannot
-/// silently narrow or widen (rustfs/backlog#1306).
-fn data_usage_info_gate_actions() -> Vec<Action> {
+/// Authorization gate for GET datausageinfo (and prefix usage): any-of the
+/// dedicated admin action OR the bucket listing action. Pinned by a unit test
+/// so the gate cannot silently narrow or widen (rustfs/backlog#1306).
+pub(crate) fn data_usage_info_gate_actions() -> Vec<Action> {
     vec![
         Action::AdminAction(AdminAction::DataUsageInfoAdminAction),
         Action::S3Action(S3Action::ListBucketAction),