优化生产日志体积：访问降噪、压缩清理与退款告警节流

跳过轮询/封面等热路径成功请求，按日 gzip 并保留可配天数，退款 max-retry 汇总告警。

backend/internal/logging/daily_writer_test.go

@@ -0,0 +1,66 @@
+package logging
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+	"time"
+)
+
+func TestCompressDayAndPurge(t *testing.T) {
+	dir := t.TempDir()
+	loc := time.FixedZone("Asia/Shanghai", 8*60*60)
+	w := newDailyWriter(dir, "app", loc, 2)
+
+	// 准备三天明文日志
+	days := []string{"2026-07-01", "2026-07-10", "2026-07-11"}
+	for _, day := range days {
+		path := filepath.Join(dir, "app-"+day+".log")
+		if err := os.WriteFile(path, []byte(`{"msg":"hello `+day+`"}`+"\n"), 0o644); err != nil {
+			t.Fatalf("write log: %v", err)
+		}
+	}
+
+	// 压缩 7-10
+	if err := w.compressDay("2026-07-10"); err != nil {
+		t.Fatalf("compressDay: %v", err)
+	}
+	if _, err := os.Stat(filepath.Join(dir, "app-2026-07-10.log")); !os.IsNotExist(err) {
+		t.Fatalf("plain log should be removed after gzip, err=%v", err)
+	}
+	if _, err := os.Stat(filepath.Join(dir, "app-2026-07-10.log.gz")); err != nil {
+		t.Fatalf("gzip missing: %v", err)
+	}
+
+	// 以 7-12 为「现在」，保留 2 天 → cutoff=7-10，删除 7-10 之前（仅 7-01）
+	now := time.Date(2026, 7, 12, 12, 0, 0, 0, loc)
+	if err := w.purgeOlderThan(now); err != nil {
+		t.Fatalf("purgeOlderThan: %v", err)
+	}
+	if _, err := os.Stat(filepath.Join(dir, "app-2026-07-01.log")); !os.IsNotExist(err) {
+		t.Fatal("expired log should be purged")
+	}
+	// 7-10 / 7-11 仍应存在（cutoff 当天起保留）
+	if _, err := os.Stat(filepath.Join(dir, "app-2026-07-10.log.gz")); err != nil {
+		t.Fatalf("recent gzip should remain: %v", err)
+	}
+	if _, err := os.Stat(filepath.Join(dir, "app-2026-07-11.log")); err != nil {
+		t.Fatalf("recent plain should remain: %v", err)
+	}
+}
+
+func TestCompressDayIdempotent(t *testing.T) {
+	dir := t.TempDir()
+	loc := time.UTC
+	w := newDailyWriter(dir, "app", loc, 0)
+	path := filepath.Join(dir, "app-2026-07-11.log")
+	if err := os.WriteFile(path, []byte("line\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if err := w.compressDay("2026-07-11"); err != nil {
+		t.Fatal(err)
+	}
+	if err := w.compressDay("2026-07-11"); err != nil {
+		t.Fatalf("second compress should be no-op: %v", err)
+	}
+}