* perf(traffic): skip unchanged global writes between heartbeats
Persist directional usage changes immediately, including lower resets, while
refreshing identical global totals only at an inclusive one-hour heartbeat.
Use the GORM clock consistently for the existing 24-hour freshness window and
preserve local-only snapshots, quota enforcement, and downstream forwarding.
Fixes#6928
* docs(traffic): describe persisted global heartbeat timestamps
Replace the stale every-push model comment with the change-or-hourly-heartbeat
rule. Keep the local-counter separation, delta accounting, MAX fold and reset
semantics explicit without treating UpdatedAt as a last-push timestamp.
Address the nonblocking review feedback on PR #6931 without rewriting
published history.
Refs #6928
Hamed