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/server/helpers_test.go

@@ -14,6 +14,7 @@ import (
 	"music-server/internal/config"
 	"music-server/internal/db/repository"
 	"music-server/internal/logging"
+	"music-server/internal/server/middleware"
 	"music-server/internal/testutil/fakedb"
 
 	"github.com/stretchr/testify/require"
@@ -48,6 +49,8 @@ type testEnv struct {
 	db      *fakeDatabase
 	server  *Server
 	handler http.Handler
+	// queueID is sent as X-Queue-Id and updated from responses, like a client keeping its queue.
+	queueID string
 }
 
 func writeFile(t *testing.T, path, content string) {
@@ -111,10 +114,16 @@ func (e *testEnv) doBody(method, path, body string, headers ...string) *httptest
 	if body != "" {
 		req.Header.Set("Content-Type", "application/json")
 	}
+	if e.queueID != "" {
+		req.Header.Set(middleware.QueueHeader, e.queueID)
+	}
 	for i := 0; i+1 < len(headers); i += 2 {
 		req.Header.Set(headers[i], headers[i+1])
 	}
 	rec := httptest.NewRecorder()
 	e.handler.ServeHTTP(rec, req)
+	if id := rec.Header().Get(middleware.QueueHeader); id != "" {
+		e.queueID = id
+	}
 	return rec
 }