feat(backend): immutable cache dir scheme + stale sweep

backend/internal/bookfile/cache.go

@@ -0,0 +1,37 @@
+package bookfile
+
+import (
+	"errors"
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+)
+
+func DirKey(id int64, hash string) string { return fmt.Sprintf("%d-%s", id, hash) }
+
+func CoverDir(cacheDir, key string) string { return filepath.Join(cacheDir, "covers", key) }
+func PagesDir(cacheDir, key string) string { return filepath.Join(cacheDir, "pages", key) }
+
+func SweepStale(cacheDir string, live map[string]bool) (int, error) {
+	removed := 0
+	for _, kind := range []string{"covers", "pages"} {
+		entries, err := os.ReadDir(filepath.Join(cacheDir, kind))
+		if err != nil {
+			if errors.Is(err, fs.ErrNotExist) {
+				continue // 首次运行还没有该层目录,不算错
+			}
+			return removed, err
+		}
+		for _, e := range entries {
+			if !e.IsDir() || live[e.Name()] {
+				continue
+			}
+			if err := os.RemoveAll(filepath.Join(cacheDir, kind, e.Name())); err != nil {
+				return removed, err
+			}
+			removed++
+		}
+	}
+	return removed, nil
+}