perf(traffic): skip unchanged global writes between heartbeats (#6931)

* 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

internal/database/model/client_global_traffic.go

@@ -1,15 +1,7 @@
 package model
 
-// ClientGlobalTraffic mirrors a master panel's aggregated (global) usage for a
-// client hosted on this panel. Masters push one row per (master, email) so the
-// node can display the client's true cross-panel total and enforce its quota
-// locally. The values never feed back into client_traffics — that table keeps
-// this panel's local-only counters, which is what keeps every master's
-// delta-baseline accounting over our snapshot correct.
-//
-// Rows are overwritten in place on every push (not max-merged), so a traffic
-// reset on the master propagates here within one push cycle. Readers that need
-// a single number fold the per-master rows with MAX.
+// ClientGlobalTraffic separates each master's aggregate from local client_traffics for delta accounting; readers fold with MAX.
+// Changed totals (including resets) overwrite on the next push; UpdatedAt tracks changes or hourly unchanged heartbeats.
 type ClientGlobalTraffic struct {
 	Id         int    `json:"id" gorm:"primaryKey;autoIncrement"`
 	MasterGuid string `json:"masterGuid" gorm:"uniqueIndex:idx_master_email,priority:1;not null"`