Keep variant content types in memory for cache hits (closes #70)

Cache.metaCache was declared and never used, so every cache hit read
and parsed the variant's .meta file. It is now an LRU
(github.com/hashicorp/golang-lru/v2) of up to 10,000 variants' content
types, filled by StoreVariant and by a read of a .meta file, so a hit
for a variant it holds skips the .meta read. Only a type from a .meta
file or a store ever enters memory, never the application/octet-stream
fallback, and a stored type is never replaced by an older one from
disk. The variant file itself is still opened on every hit, so nothing
is served from memory alone.

Model: opus-5-5

internal/imgcache/storage.go

@@ -506,32 +506,44 @@ func (s *VariantStorage) Load(key VariantKey) (io.ReadCloser, error) {
 	return f, nil
 }
 
-// LoadWithMeta returns a reader, size, and content type for the content at
-// the given key.
-func (s *VariantStorage) LoadWithMeta(
-	key VariantKey,
-) (io.ReadCloser, int64, string, error) {
+// LoadWithSize returns a reader and file size for the content at the
+// given key.
+func (s *VariantStorage) LoadWithSize(key VariantKey) (io.ReadCloser, int64, error) {
 	path := s.keyToPath(key)
-	metaPath := path + ".meta"
 
 	f, err := os.Open(path) //nolint:gosec // path derived from cache key
 	if err != nil {
 		if os.IsNotExist(err) {
-			return nil, 0, "", ErrNotFound
+			return nil, 0, ErrNotFound
 		}
 
-		return nil, 0, "", fmt.Errorf("failed to open content: %w", err)
+		return nil, 0, fmt.Errorf("failed to open content: %w", err)
 	}
 
 	stat, err := f.Stat()
 	if err != nil {
 		_ = f.Close()
 
-		return nil, 0, "", fmt.Errorf("failed to stat content: %w", err)
+		return nil, 0, fmt.Errorf("failed to stat content: %w", err)
 	}
 
-	// Load metadata for content type
-	contentType := "application/octet-stream" // fallback
+	return f, stat.Size(), nil
+}
+
+// LoadWithMeta returns a reader, size, and content type for the content at
+// the given key. The content type is read from the .meta file, and is
+// empty when that file is missing or unreadable.
+func (s *VariantStorage) LoadWithMeta(
+	key VariantKey,
+) (io.ReadCloser, int64, string, error) {
+	f, size, err := s.LoadWithSize(key)
+	if err != nil {
+		return nil, 0, "", err
+	}
+
+	var contentType string
+
+	metaPath := s.keyToPath(key) + ".meta"
 
 	metaData, err := os.ReadFile(metaPath) //nolint:gosec // path derived from cache key
 	if err == nil {
@@ -541,7 +553,7 @@ func (s *VariantStorage) LoadWithMeta(
 		}
 	}
 
-	return f, stat.Size(), contentType, nil
+	return f, size, contentType, nil
 }
 
 // Exists checks if content exists at the given key.