Store each client's play queue in the database

The queue lived in memory and was shared by all clients, so a restart or a
finished sync cleared it. Every client now has its own queue in the new
queue and queue_song tables (added to the undeployed migration 000006).

The music endpoints return the queue ID in the X-Queue-Id header and expect
it back. A request without an ID, or with an ended one, starts a new queue.
/music/reset ends the queue and returns a new ID, and a sync ends all
running queues. Ended queues are kept as history.

The deprecated legacy routes without the /api/v1 prefix are removed.

internal/backend/queue_integration_test.go

@@ -0,0 +1,89 @@
+//go:build integration
+
+package backend
+
+import (
+	"context"
+	"testing"
+
+	"music-server/internal/db/repository"
+	"music-server/internal/testutil/pgtest"
+
+	"github.com/google/uuid"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestQueueWithPostgres checks the queue queries against a real database.
+func TestQueueWithPostgres(t *testing.T) {
+	ctx := context.Background()
+	database := pgtest.NewMigratedDatabase(t)
+	q := repository.New(database.Pool)
+	s := NewSyncService(q, copyTestMusic(t), nil)
+	require.True(t, s.Start(false))
+	s.Wait()
+
+	m := NewMusicService(q, t.TempDir())
+	queueID, err := m.ResolveQueue(ctx, "")
+	require.NoError(t, err)
+
+	require.NoError(t, m.AddLatestToQueue(ctx, queueID), "nothing fetched yet")
+	var paths []string
+	for range 3 {
+		path, err := m.RandomSong(ctx, queueID)
+		require.NoError(t, err)
+		require.NoError(t, m.AddLatestToQueue(ctx, queueID))
+		paths = append(paths, path)
+	}
+	require.NoError(t, m.AddLatestToQueue(ctx, queueID), "a fetched song is only added once")
+
+	played, err := m.PlayedSongs(ctx, queueID)
+	require.NoError(t, err)
+	require.Len(t, played, 3)
+	for i, song := range played {
+		assert.Equal(t, i, song.SongNo)
+		assert.Equal(t, i == 2, song.CurrentlyPlaying, "the last added song is current")
+		assert.NotEmpty(t, song.Soundtrack)
+		assert.NotEqual(t, uuid.Nil, song.SoundtrackUuid)
+	}
+
+	path, err := m.PreviousSong(ctx, queueID)
+	require.NoError(t, err)
+	assert.Equal(t, paths[1], path)
+	require.NoError(t, m.AddLatestPlayed(ctx, queueID))
+	info, err := m.SongInfo(ctx, queueID)
+	require.NoError(t, err)
+	assert.Equal(t, 1, info.SongNo)
+	assert.Equal(t, int32(1), info.SongPlayed)
+
+	// A new service, as after a restart, sees the same queue.
+	restarted := NewMusicService(q, t.TempDir())
+	same, err := restarted.ResolveQueue(ctx, queueID.String())
+	require.NoError(t, err)
+	assert.Equal(t, queueID, same)
+
+	newID, err := m.NewQueue(ctx, queueID)
+	require.NoError(t, err)
+	assert.NotEqual(t, queueID, newID)
+	resolved, err := m.ResolveQueue(ctx, queueID.String())
+	require.NoError(t, err)
+	assert.NotEqual(t, queueID, resolved, "an ended queue is not used again")
+
+	other, err := m.ResolveQueue(ctx, "")
+	require.NoError(t, err)
+	require.NoError(t, m.EndAllQueues(ctx))
+	var active int
+	require.NoError(t, database.Pool.QueryRow(ctx, "SELECT COUNT(*) FROM queue WHERE ended_at IS NULL").Scan(&active))
+	assert.Zero(t, active)
+	_, err = m.ResolveQueue(ctx, other.String())
+	require.NoError(t, err)
+
+	// Ended queues keep their songs, also after the songs are removed from the library.
+	require.NoError(t, q.ClearSongs(ctx))
+	played, err = m.PlayedSongs(ctx, queueID)
+	require.NoError(t, err)
+	require.Len(t, played, 3)
+	assert.NotEmpty(t, played[0].Song)
+	_, err = m.Song(ctx, queueID, 0)
+	assert.ErrorIs(t, err, ErrNotFound)
+}