Generate the OpenAPI spec with Huma instead of swag

The spec is now built from the same calls that register the routes, so it
can't drift from the router like the swag comments did (all /api/v1
routes were missing).

- /api/v1 endpoints are typed Huma operations; the spec is OpenAPI 3.1 at
  /openapi.json (3.0 at /openapi-3.0.json, YAML too), docs at /docs.
  /openapi and /swagger/* redirect there
- Bearer token security is declared per operation; token creation and
  the new GET /api/v1/health are public
- Legacy routes use the same handlers, are hidden from the spec and keep
  the Deprecation header
- /sync/reset is hidden (debug only); the /sync/new, /sync/quick and
  /sync/new/full aliases are removed (use /sync and /sync/full)
- Invalid parameters return 422 (validation); every error is
  {"error": "..."}; empty lists are [] instead of null
- /sync/progress is documented as progress or result (oneOf)
- Tests validate the spec and check real responses of every documented
  operation against it (kin-openapi)
- Removed swag annotations, cmd/docs, echo-swagger and the swag tool

internal/backend/statistics.go

@@ -66,7 +66,7 @@ func toSoundtracks[R soundtrackRow](rows []R, err error) ([]SoundtrackWithSongs,
 	if err != nil {
 		return nil, err
 	}
-	var result []SoundtrackWithSongs
+	result := []SoundtrackWithSongs{}
 	for _, r := range rows {
 		row := repository.GetMostPlayedSoundtracksWithSongsRow(r)
 		result = append(result, SoundtrackWithSongs{
@@ -89,7 +89,7 @@ func toSongs[R songRow](rows []R, err error) ([]SongInfoForStats, error) {
 	if err != nil {
 		return nil, err
 	}
-	var result []SongInfoForStats
+	result := []SongInfoForStats{}
 	for _, r := range rows {
 		result = append(result, SongInfoForStats(repository.GetMostPlayedSongsWithSoundtrackRow(r)))
 	}
@@ -132,7 +132,7 @@ func (s *StatisticsService) NeverPlayedSoundtracks(ctx context.Context) ([]Sound
 	if err != nil {
 		return nil, err
 	}
-	var result []SoundtrackWithSongs
+	result := []SoundtrackWithSongs{}
 	for _, row := range rows {
 		result = append(result, SoundtrackWithSongs{
 			SoundtrackID:     row.SoundtrackID,