feat: add global system notices and Beijing order time display

internal/store/time.go

@@ -0,0 +1,8 @@
+package store
+
+// databaseWallTimeUTC normalizes DATETIME values written by CURRENT_TIMESTAMP.
+// The database session can use UTC or Beijing time independently of the Go process.
+// Do not use it for fields explicitly written as UTC by application code.
+func databaseWallTimeUTC(column string) string {
+	return `DATE_FORMAT(TIMESTAMPADD(SECOND,TIMESTAMPDIFF(SECOND,NOW(),UTC_TIMESTAMP()),` + column + `),'%Y-%m-%dT%H:%i:%sZ')`
+}