fix(scanner): fit usage persistence within publication lease (#6967)

Lower the default scanner cache save timeout so the derived usage persistence budget stays inside the effective distributed publication lease window.

Add focused regressions for the default publication budget and bootstrap-pending observational baselines, and update operator docs with the new default.

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

crates/scanner/src/data_usage_define/tests.rs

@@ -2241,6 +2241,7 @@ fn test_cache_save_timeout_uses_default_when_env_missing() {
             DataUsageCache::cache_save_timeout(),
             Duration::from_secs(rustfs_config::DEFAULT_SCANNER_CACHE_SAVE_TIMEOUT_SECS)
         );
+        assert_eq!(DataUsageCache::persistence_timeout(), Duration::from_millis(52_350));
     });
     crate::runtime_config::refresh_scanner_runtime_config_for_tests();
 }