Disk cache: version the poster folder

Posters stored by the previous build (the downloaded bytes, for real
images Radarr's 333x500 copy) were reused as they were for 30 days.
Posters now live in posters-v2/; older folders are deleted at startup.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/ui/diskcache.go

@@ -18,8 +18,17 @@ const (
 	posterCacheCap = 200 << 20           // bytes of posters kept on disk
 	peopleTTL      = 7 * 24 * time.Hour  // a cast on disk is used this long
 	maxPeopleSaved = 2000                // casts kept (in memory and on disk)
+
+	// postersDir names the poster folder for the current way of storing them
+	// (v2: as decoded, at the size used). Folders of older ways are deleted by
+	// prunePosters: their files would otherwise be used as they are (v1 kept
+	// Radarr's 333×500 copies for real images).
+	postersDir = "posters-v2"
 )
 
+// oldPosterDirs are poster folders of earlier versions.
+var oldPosterDirs = []string{"posters"}
+
 // cacheDirFunc returns the directory for the disk cache, "" for none. It is a
 // variable so tests can turn the cache off.
 var cacheDirFunc = func() string {
@@ -42,7 +51,7 @@ func (d *diskCache) posterPath(key string, large bool) string {
 		size = "large"
 	}
 	sum := sha256.Sum256([]byte(size + "|" + key))
-	return filepath.Join(d.dir, "posters", hex.EncodeToString(sum[:16])+".img")
+	return filepath.Join(d.dir, postersDir, hex.EncodeToString(sum[:16])+".img")
 }
 
 // readPoster returns the stored bytes of a poster, if they are not too old.
@@ -85,9 +94,12 @@ func (d *diskCache) prunePosters() {
 		size int64
 		mod  time.Time
 	}
+	for _, old := range oldPosterDirs {
+		_ = os.RemoveAll(filepath.Join(d.dir, old))
+	}
 	var files []file
 	var total int64
-	_ = filepath.WalkDir(filepath.Join(d.dir, "posters"), func(p string, e fs.DirEntry, err error) error {
+	_ = filepath.WalkDir(filepath.Join(d.dir, postersDir), func(p string, e fs.DirEntry, err error) error {
 		if err != nil || e.IsDir() {
 			return nil
 		}

internal/ui/diskcache_test.go

@@ -105,3 +105,19 @@ func TestPeopleCacheKeepsTheNewest(t *testing.T) {
 		t.Error("new casts must be written")
 	}
 }
+
+func TestPostersOfOlderVersionsAreDropped(t *testing.T) {
+	d := &diskCache{dir: t.TempDir()}
+	old := filepath.Join(d.dir, "posters", "abc.img")
+	if err := writeAtomic(old, []byte("v1")); err != nil {
+		t.Fatal(err)
+	}
+	d.writePoster("k", true, []byte("v2"))
+	d.prunePosters()
+	if _, err := os.Stat(old); !os.IsNotExist(err) {
+		t.Error("a poster of the old format was kept")
+	}
+	if data, ok := d.readPoster("k", true); !ok || string(data) != "v2" {
+		t.Error("the current poster was dropped")
+	}
+}