fix: resolve all lint errors and make linting a required CI check (#62)

- Fix 10 err113 violations: extract dynamic errors to package-level sentinels
- Fix 12 errcheck violations: handle unchecked error returns in player,
  metadata, and config packages
- Fix 4 revive stutter warnings: rename player.PlayerState to player.State,
  player.PlayerVolume to player.Volume, queue.QueueTrack to queue.Track,
  queue.QueueState to queue.State
- Fix 2 staticcheck SA4001: simplify *&x to x in assets handler
- Fix 5 unused constants: remove dead AudioFileType iota block in models
- Fix gci/gofumpt/wsl formatting issues across multiple files
- Add gofumpt module-path setting to .golangci.yml for correct import grouping
- Fix player test: gate integration test behind YELLOWJACKET_INTEGRATION env var
  instead of only skipping in CI, and replace t.Errorf+t.Failed with t.Fatalf
- Remove continue-on-error from golangci-lint CI step so linting is now required

backend/metadata/metadata.go

@@ -39,14 +39,14 @@ func GetTrackLengthMillis(path string) (int64, error) {
 
 	streamer, format, err := DecodeFile(f)
 	if err != nil {
-		f.Close()
+		_ = f.Close()
 
 		return 0, fmt.Errorf("error decoding file: %w", err)
 	}
 
 	lengthMillis := int64(float64(streamer.Len()*1000) / float64(format.SampleRate))
-	streamer.Close()
-	f.Close()
+	_ = streamer.Close()
+	_ = f.Close()
 
 	return lengthMillis, nil
 }

backend/metadata/tags.go

@@ -51,7 +51,8 @@ func ExtractTags(path string) (*TrackMetadata, error) {
 	if err != nil {
 		return nil, fmt.Errorf("could not open file for tag extraction: %w", err)
 	}
-	defer f.Close()
+
+	defer func() { _ = f.Close() }()
 
 	return ExtractTagsFromReader(f)
 }