Rewrite tests to run without a database

- Unit tests use an in-memory fake database (internal/testutil/fakedb),
  temp folders and httptest; `go test ./...` needs no Docker or env vars
- Integration tests (build tag "integration") start PostgreSQL with
  testcontainers: migrations step by step, SQL dumps (skipped if
  missing) and a full sync against a real database
- Removed tests that only checked copies of the logic or had empty
  bodies, the broken root integration_test.go, compose.test.yaml and
  .env.test with hardcoded paths
- testMusic MP3s replaced with tiny placeholders (sync never decodes
  audio), 194 MB -> 168 KB

internal/backend/sync_integration_test.go

@@ -0,0 +1,111 @@
+//go:build integration
+
+package backend
+
+import (
+	"context"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"music-server/internal/db/repository"
+	"music-server/internal/logging"
+	"music-server/internal/testutil/pgtest"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestMain(m *testing.M) {
+	logging.Init("fatal", false)
+	os.Exit(pgtest.Main(m))
+}
+
+// copyTestMusic copies the repository's testMusic folder so tests can change it.
+func copyTestMusic(t *testing.T) string {
+	t.Helper()
+	dst := t.TempDir()
+	require.NoError(t, os.CopyFS(dst, os.DirFS(filepath.Join("..", "..", "testMusic"))))
+	return dst
+}
+
+// TestSyncWithPostgres runs syncs against a real database to check the SQL queries.
+func TestSyncWithPostgres(t *testing.T) {
+	ctx := context.Background()
+	q := repository.New(pgtest.NewMigratedDatabase(t).Pool)
+	musicDir := copyTestMusic(t)
+	s := NewSyncService(q, musicDir, nil)
+	run := func(full bool) SyncResponse {
+		require.True(t, s.Start(full))
+		s.Wait()
+		result := s.Result()
+		require.Empty(t, result.CatchedErrors)
+		return result
+	}
+
+	result := run(false)
+	assert.Equal(t, []string{"A Bird Story", "Clair Obscur - Expedition 33", "Metal Gear Rising - Revengeance"}, result.SoundtracksAdded)
+
+	soundtracks, err := q.FindAllSoundtracks(ctx)
+	require.NoError(t, err)
+	require.Len(t, soundtracks, 3)
+	bird := soundtracks[0]
+	assert.NotEmpty(t, bird.Fingerprint)
+	songs, err := q.FindSongsFromSoundtrack(ctx, bird.ID)
+	require.NoError(t, err)
+	assert.Len(t, songs, 6)
+
+	// Play a song, then rename one of its files and, in a later sync, the soundtrack folder.
+	// (Renaming both at once changes the content hash, so the folder would count as new.)
+	require.NoError(t, q.AddSongPlayed(ctx, repository.AddSongPlayedParams{SoundtrackID: bird.ID, SongName: "2. Yesterday"}))
+	birdDir := filepath.Join(musicDir, "A Bird Story")
+	require.NoError(t, os.Rename(filepath.Join(birdDir, "2. Yesterday.mp3"), filepath.Join(birdDir, "02 Yesterday.mp3")))
+
+	result = run(false)
+	assert.Equal(t, []string{"A Bird Story"}, result.SoundtracksChangedContent)
+
+	require.NoError(t, os.Rename(birdDir, filepath.Join(musicDir, "A Bird Story (2014)")))
+	result = run(false)
+	assert.Equal(t, map[string]string{"A Bird Story": "A Bird Story (2014)"}, result.SoundtracksChangedTitle)
+	assert.Empty(t, result.SoundtracksAdded)
+	assert.Empty(t, result.SoundtracksRemoved)
+
+	renamed, err := q.GetSoundtrackById(ctx, bird.ID)
+	require.NoError(t, err)
+	assert.Equal(t, "A Bird Story (2014)", renamed.SoundtrackName)
+	songs, err = q.FindSongsFromSoundtrack(ctx, bird.ID)
+	require.NoError(t, err)
+	require.Len(t, songs, 6)
+	var yesterday repository.Song
+	for _, song := range songs {
+		assert.Contains(t, song.Path, "/A Bird Story (2014)/")
+		if song.SongName == "02 Yesterday" {
+			yesterday = song
+		}
+	}
+	assert.Equal(t, int32(1), yesterday.TimesPlayed, "play count survives renames")
+
+	// Nothing changed: the fingerprint lets the sync skip everything.
+	result = run(false)
+	assert.Empty(t, result.SoundtracksChangedContent)
+	assert.Empty(t, result.SoundtracksChangedTitle)
+
+	// Remove a soundtrack and bring it back.
+	mgr := filepath.Join(musicDir, "Metal Gear Rising - Revengeance")
+	backup := filepath.Join(t.TempDir(), "mgr")
+	require.NoError(t, os.Rename(mgr, backup))
+	result = run(false)
+	assert.Equal(t, []string{"Metal Gear Rising - Revengeance"}, result.SoundtracksRemoved)
+	active, err := q.FindAllSoundtracks(ctx)
+	require.NoError(t, err)
+	assert.Len(t, active, 2)
+
+	require.NoError(t, os.Rename(backup, mgr))
+	result = run(true)
+	assert.Equal(t, []string{"Metal Gear Rising - Revengeance"}, result.SoundtracksReAdded)
+
+	// Classic random picks only songs of active soundtracks.
+	row, err := q.GetRandomActiveSong(ctx)
+	require.NoError(t, err)
+	assert.NotEmpty(t, row.SoundtrackPath)
+}