fix: resolve all 16 lint failures — make check passes clean

Fixed issues:
- gochecknoglobals: moved vipsOnce into ImageProcessor struct field
- gosec G703 (path traversal): added nolint for hash-derived paths (matching existing pattern)
- gosec G704 (SSRF): added URL validation (scheme + host) before HTTP request
- gosec G306: changed file permissions from 0640 to named constant StorageFilePerm (0600)
- nlreturn: added blank lines before 7 return statements
- revive unused-parameter: renamed unused 'groups' parameter to '_'
- unused field: removed unused metaCacheMu from Cache struct

Note: gosec G703/G704 taint analysis traces data flow from function parameters
through all operations. No code-level sanitizer (filepath.Clean, URL validation,
hex validation) breaks the taint chain. Used nolint:gosec matching the existing
pattern in storage.go for the same false-positive class (paths derived from
SHA256 content hashes, not user input).

internal/config/config.go

@@ -132,7 +132,8 @@ func loadConfigFile(log *slog.Logger, appName string) (*smartconfig.Config, erro
 	}
 
 	for _, path := range configPaths {
-		if _, statErr := os.Stat(path); statErr == nil {
+		path = filepath.Clean(path)
+		if _, statErr := os.Stat(path); statErr == nil { //nolint:gosec // paths are hardcoded config locations
 			sc, err := smartconfig.NewFromConfigPath(path)
 			if err != nil {
 				log.Warn("failed to parse config file", "path", path, "error", err)