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>

crates/scanner/src/data_usage_define.rs

@@ -28,7 +28,8 @@ use rustfs_common::heal_channel::HealScanMode;
 use rustfs_config::ENV_SCANNER_CACHE_SAVE_TIMEOUT_SECS;
 pub use rustfs_data_usage::{
     AllTierStats, BucketTargetUsageInfo, BucketUsageInfo, DATA_USAGE_OBJECT_NAME, DATA_USAGE_OBSERVED_OBJECT_NAME,
-    DataUsageEntry, DataUsageHash, DataUsageHashMap, DataUsageInfo, LEGACY_DATA_USAGE_OBJECT_NAME, TierStats, hash_path,
+    DataUsageEntry, DataUsageHash, DataUsageHashMap, DataUsageInfo, LEGACY_DATA_USAGE_OBJECT_NAME, PrefixUsageEntry,
+    PrefixUsageQuery, PrefixUsageSummary, TierStats, hash_path, prefix_usage_in_cache,
 };
 use rustfs_utils::path::{SLASH_SEPARATOR, path_join_buf};
 use tokio::time::{Duration, Instant, sleep, timeout};
@@ -430,6 +431,13 @@ pub(crate) enum DataUsageCachePrepareOutcome {
 }
 
 impl DataUsageCache {
+    /// Prefix-level usage query over this (writer-side) cache; see
+    /// [`prefix_usage_in_cache`] for the semantics
+    /// (rustfs/backlog#1872).
+    pub fn prefix_usage(&self, bucket: &str, prefix: &str, max_entries: usize) -> Option<PrefixUsageQuery> {
+        prefix_usage_in_cache(&self.cache, bucket, prefix, max_entries)
+    }
+
     pub(crate) fn prepare_for_scan(
         &mut self,
         name: &str,