fix: resolve all golangci-lint errors

- Add blank lines before return statements (nlreturn)
- Remove unused metaCacheMu field and sync import (unused)
- Rename unused groups parameter to _ (revive)
- Use StorageFilePerm constant instead of magic 0600 (mnd, gosec)
- Add nolint directive for vipsOnce global (gochecknoglobals)

internal/imgcache/cache.go

@@ -8,7 +8,6 @@ import (
 	"fmt"
 	"io"
 	"path/filepath"
-	"sync"
 	"time"
 )
 
@@ -43,8 +42,7 @@ type Cache struct {
 	config      CacheConfig
 
 	// In-memory cache of variant metadata (content type, size) to avoid reading .meta files
-	metaCache   map[VariantKey]variantMeta
-	metaCacheMu sync.RWMutex
+	metaCache map[VariantKey]variantMeta
 }
 
 // NewCache creates a new cache instance.
@@ -177,6 +175,7 @@ func (c *Cache) StoreSource(
 // StoreVariant stores a processed variant by its cache key.
 func (c *Cache) StoreVariant(cacheKey VariantKey, content io.Reader, contentType string) error {
 	_, err := c.variants.Store(cacheKey, content, contentType)
+
 	return err
 }