fix(site-replication): persist source stamps and apply IAM items atomically (backlog#2291)

Review findings on rustfs#7195: the receive-side staleness gate compared a
source `updatedAt` against a stamp the local write had put on the record,
and the verdict, the write and the deletion mark were three separate steps.

- IAM writes gain explicit-stamp variants (`set_policy_at`, `policy_db_set_at`,
  group and user `*_at`, `new_service_account_at`, `update_service_account_at`)
  so a replicated record carries its source time; local edits are unchanged.
- `apply_iam_item` runs verdict, write and mark commit under the
  site-replication state transaction (distributed state-object lock), so a
  concurrent older grant and newer revoke are ordered on every node.
- A replicated service account is created with its source status in one
  write (`NewServiceAccountOpts::status`), never enabled transiently.
- Deletion marks are pruned by age (30 days) instead of by count.
- Bucket-config deletes persist the source stamp (`delete_if_incarnation_at`).

Regressions run through the real receiver: delayed in-order updates for every
gated item type, concurrent grant/revoke, delete then stale re-create, disabled
service-account create, delete stamping in ecstore, and mark retention.

crates/policy/src/policy/doc.rs

@@ -45,18 +45,33 @@ pub struct PolicyDoc {
 
 impl PolicyDoc {
     pub fn new(policy: Policy) -> Self {
+        Self::new_at(policy, OffsetDateTime::now_utc())
+    }
+
+    /// [`Self::new`] with an explicit `UpdateDate` (and `CreateDate`).
+    ///
+    /// A replicated document keeps the edit's source time: the receiver
+    /// judges the next incoming revision against the stored stamp, so a
+    /// local stamp would reject a newer source edit that was merely
+    /// delivered later.
+    pub fn new_at(policy: Policy, at: OffsetDateTime) -> Self {
         Self {
             version: 1,
             policy,
-            create_date: Some(OffsetDateTime::now_utc()),
-            update_date: Some(OffsetDateTime::now_utc()),
+            create_date: Some(at),
+            update_date: Some(at),
         }
     }
 
     pub fn update(&mut self, policy: Policy) {
+        self.update_at(policy, OffsetDateTime::now_utc());
+    }
+
+    /// [`Self::update`] with an explicit `UpdateDate`; see [`Self::new_at`].
+    pub fn update_at(&mut self, policy: Policy, at: OffsetDateTime) {
         self.version += 1;
         self.policy = policy;
-        self.update_date = Some(OffsetDateTime::now_utc());
+        self.update_date = Some(at);
 
         if self.create_date.is_none() {
             self.create_date = self.update_date;