feat(scanner): emit excess alerts as S3 notification events

The excess-versions / excess-version-size / excess-folders alerts were
metrics-and-logs only; consoles and external auditors had no way to hear
them (rustfs/backlog#1868, HS-04). MinIO emits s3:ObjectManyVersions /
s3:ObjectLargeVersions / s3:PrefixManyFolders for the same conditions —
RustFS carries those as EventName::Scanner* with s3:Scanner:* wire names
that already existed unpublished.

The three alert sites now also dispatch through the standard event
pipeline (send_event via the storage_api owner facade), carrying the
actual values and thresholds in req_params and UserAgent "Scanner".
Without a cooldown a single over-threshold object would re-emit on every
~60s scan cycle, so emissions are edge-held per (kind, bucket, object)
for 24h (RUSTFS_SCANNER_ALERT_COOLDOWN_SECS, 0 = every cycle), backed by
a process-global map with a 4096-key hard cap that clears rather than
grows. Metrics and structured logs stay level-triggered every cycle;
only the notification events are held back. A restart resets the
cooldown deliberately: one re-emission per still-hot key buys back
visibility after the restarts that accompany incident response.

Tests pin the edge-hold semantics (first fires, immediate re-check held,
independent keys, cooldown expiry re-fires, zero cooldown always emits,
hard bound) in one sequential test for the process-global map, and pin
the emitted wire names against EventName's canonical string forms so a
subscribed bucket notification can never silently stop matching.
docs/operations/scanner-excess-alerts.md documents the three events,
the metric-vs-event cadence difference, and the HS-15 threshold deltas
(alert_excess_folders 65538 vs MinIO 50000 is deliberate: Proxmox
Backup Server chunk layout compatibility).

Closes rustfs/backlog#1868.

Co-Authored-By: heihutu <heihutu@gmail.com>

Cargo.lock

@@ -10251,6 +10251,7 @@ dependencies = [
  "rustfs-ecstore",
  "rustfs-filemeta",
  "rustfs-lock",
+ "rustfs-s3-types",
  "rustfs-storage-api",
  "rustfs-utils",
  "s3s",