diag: log slow QSO inserts (>2s) with call/band/mode; rotate the app log to .1 at 10MB instead of deleting it, so the previous session's diagnostics survive

internal/applog/applog.go

@@ -47,10 +47,15 @@ func Init(dataDir string) (string, error) {
 			_ = os.Rename(oldLog, logPath)
 		}
 	}
-	// Truncate if the file grew past ~5MB so we don't accumulate logs
-	// forever. We keep one file — simple and adequate for diagnostics.
-	if fi, err := os.Stat(logPath); err == nil && fi.Size() > 5*1024*1024 {
-		_ = os.Remove(logPath)
+	// Rotate (don't delete) once the file grows past ~10MB: rename it to
+	// opslog.log.1 so the PREVIOUS session's log survives. Deleting it outright used
+	// to erase exactly the diagnostics we needed when a user reported an issue from
+	// the session that just ended. One generation kept — enough, without unbounded growth.
+	if fi, err := os.Stat(logPath); err == nil && fi.Size() > 10*1024*1024 {
+		_ = os.Remove(logPath + ".1")     // drop the older generation
+		if err := os.Rename(logPath, logPath+".1"); err != nil {
+			_ = os.Remove(logPath) // rename failed (locked?) → fall back to the old behaviour
+		}
 	}
 	f, err := os.OpenFile(logPath, os.O_CREATE|os.O_WRONLY|os.O_APPEND, 0o644)
 	if err != nil {