Add BT1035 pairing and station presets (fw 0.7.0).

Expose discoverable mode and A2DP control over REST, add persisted
DAB/FM preset list with web UI, and document gaps in docs/TODO.md.

Co-authored-by: Cursor <cursoragent@cursor.com>

Software/components/core/src/BluetoothJson.cpp

@@ -0,0 +1,36 @@
+/**
+ * @file    BluetoothJson.cpp
+ * @brief   BluetoothJson implementation.
+ *
+ * DigiRadio firmware — https://github.com/manvalan/DigiRadio
+ *
+ * Copyright 2026 Michele Bigi
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * @author  Michele Bigi
+ * @date    2026-07-06
+ */
+
+#include "core/BluetoothJson.hpp"
+
+#include <sstream>
+
+namespace core {
+
+std::string serializeBluetoothStatusJson(const BluetoothStatus& status)
+{
+    std::ostringstream out;
+    out << "{\"booted\":" << (status.booted ? "true" : "false")
+        << ",\"pairing\":" << (status.pairing ? "true" : "false")
+        << ",\"a2dp\":\"" << a2dpStateToken(status.a2dpState) << "\"}";
+    return out.str();
+}
+
+std::string serializeBluetoothErrorJson(const char* reason)
+{
+    std::ostringstream out;
+    out << "{\"status\":\"error\",\"reason\":\"" << reason << "\"}";
+    return out.str();
+}
+
+} // namespace core