Prefer UUIDs in the API (Phase 3 of the UUID migration)

- New /api/v1 endpoints: GET /soundtracks, /soundtracks/{uuid},
  /songs/{uuid} and /songs/{uuid}/file (streams the song, Range support,
  locked during sync)
- UUIDs added to every response with soundtracks or songs: queue info and
  list (SoundtrackUuid, SongUuid) and statistics (soundtrack_uuid,
  song_uuid). Existing fields stay for backward compatibility
- Play counts and broken-song removal use song UUIDs instead of
  soundtrack ID + song name, so songs with the same name are counted
  separately
- Statistics return an empty song list for soundtracks without songs
  instead of a single empty entry
- The legacy .id file cleanup stays until Phase 4 (#31)

Closes #29

internal/backend/statistics.go

@@ -6,25 +6,31 @@ import (
 	"time"
 
 	"music-server/internal/db/repository"
+
+	"github.com/google/uuid"
 )
 
 // SoundtrackWithSongs represents a soundtrack with its songs for statistics
 type SoundtrackWithSongs struct {
 	SoundtrackID         int32              `json:"soundtrack_id"`
+	SoundtrackUuid       uuid.UUID          `json:"soundtrack_uuid"`
 	SoundtrackName       string             `json:"soundtrack_name"`
 	SoundtrackPlayed     int32              `json:"soundtrack_played"`
 	SoundtrackLastPlayed *time.Time         `json:"soundtrack_last_played,omitempty"`
 	Songs                []SongInfoForStats `json:"songs"`
 }
 
-// SongInfoForStats represents a song with soundtrack info for statistics
+// SongInfoForStats represents a song with soundtrack info for statistics.
+// The field order matches the generated query rows so they can be converted directly.
 type SongInfoForStats struct {
-	SoundtrackID   int32   `json:"soundtrack_id"`
-	SoundtrackName string  `json:"soundtrack_name"`
-	SongName       string  `json:"song_name"`
-	Path           string  `json:"path"`
-	TimesPlayed    int32   `json:"times_played"`
-	FileName       *string `json:"file_name,omitempty"`
+	SoundtrackID   int32     `json:"soundtrack_id"`
+	SoundtrackUuid uuid.UUID `json:"soundtrack_uuid"`
+	SongUuid       uuid.UUID `json:"song_uuid"`
+	SoundtrackName string    `json:"soundtrack_name"`
+	SongName       string    `json:"song_name"`
+	Path           string    `json:"path"`
+	TimesPlayed    int32     `json:"times_played"`
+	FileName       *string   `json:"file_name,omitempty"`
 }
 
 // StatisticsSummary holds overall statistics
@@ -65,6 +71,7 @@ func toSoundtracks[R soundtrackRow](rows []R, err error) ([]SoundtrackWithSongs,
 		row := repository.GetMostPlayedSoundtracksWithSongsRow(r)
 		result = append(result, SoundtrackWithSongs{
 			SoundtrackID:         row.SoundtrackID,
+			SoundtrackUuid:       row.SoundtrackUuid,
 			SoundtrackName:       row.SoundtrackName,
 			SoundtrackPlayed:     row.SoundtrackPlayed,
 			SoundtrackLastPlayed: row.SoundtrackLastPlayed,
@@ -129,6 +136,7 @@ func (s *StatisticsService) NeverPlayedSoundtracks(ctx context.Context) ([]Sound
 	for _, row := range rows {
 		result = append(result, SoundtrackWithSongs{
 			SoundtrackID:     row.SoundtrackID,
+			SoundtrackUuid:   row.SoundtrackUuid,
 			SoundtrackName:   row.SoundtrackName,
 			SoundtrackPlayed: row.SoundtrackPlayed,
 			Songs:            parseSongs(row.Songs),