app: PlaybackController + playhead subscription migration (issues 0c, 1-4)

New app/playback/playbackcontroller.{h,cpp}: a lazily created
process-wide singleton whose set_playhead() forwards to
oakengine_viewer_set_playhead and re-broadcasts playhead_changed as a
plain Qt signal. Every app-side oakengine_viewer_set_playhead call site
(17 across viewer, timelinewidget, timelineview, import tool,
timebasedwidget, keyframecontrol, export dialog, mainwindow) now goes
through it.

Migrate the first four playhead subscribers off the raw C event
subscription / EngineEventBridge to the controller signal:
- TimeBasedView (issue 1)
- NodeParamViewWidgetBridge (issue 2)
- NodeParamViewKeyframeControl (issue 3)
- NodeParamViewConnectedLabel (issue 4)

Each stores a QMetaObject::Connection filtered on its viewer node and
disconnects on teardown; the viewer_sub_ members are gone. Plan docs
mark issues 0c and 1-4 as done.

Closes #30, closes #31, closes #32, closes #51, closes #52.

app/playback/playbackcontroller.h

@@ -0,0 +1,50 @@
+#ifndef OAK_PLAYBACKCONTROLLER_H
+#define OAK_PLAYBACKCONTROLLER_H
+
+#include <QObject>
+
+#include "oakengine/viewer.h"
+#include "olive/core/util/rational.h"
+
+namespace olive
+{
+
+/**
+ * @brief App-internal hub for playhead changes (issue 0c of the
+ * EventBridge elimination plan).
+ *
+ * Every app-side `oakengine_viewer_set_playhead` call site goes through
+ * set_playhead(), which forwards to the engine and re-broadcasts
+ * playhead_changed as a plain Qt signal. Widgets subscribe to that signal
+ * instead of OAKENGINE_EVENT_VIEWER_PLAYHEAD_CHANGED via EngineEventBridge
+ * or raw oakengine_event_subscribe callbacks.
+ */
+class PlaybackController : public QObject {
+	Q_OBJECT
+public:
+	/**
+	 * @brief Lazily created process-wide instance (leaked at exit, like
+	 * Core). Widget tests construct widgets without Core::Start(), so the
+	 * instance must not depend on explicit startup.
+	 */
+	static PlaybackController *instance();
+
+	/**
+	 * @brief Move the playhead and notify app subscribers.
+	 */
+	void set_playhead(OakEngineNode *viewer, const core::Rational &time);
+	void set_playhead(OakEngineNode *viewer, int64_t num, int64_t den);
+
+signals:
+	void playhead_changed(OakEngineNode *viewer,
+						  const olive::core::Rational &time);
+
+private:
+	explicit PlaybackController(QObject *parent = nullptr);
+
+	static PlaybackController *instance_;
+};
+
+}
+
+#endif // OAK_PLAYBACKCONTROLLER_H