feat(tts): server-backed TTS engine selection

Introduce server TTS support and engine selection while keeping
device TTS as the default.

- Add new persistence keys for storing TTS engine and selected
  server voice (ttsEngine, ttsServerVoiceId, ttsServerVoiceName).
- Extend TextToSpeechService to support two engines:
  TtsEngine.device (FlutterTts) and TtsEngine.server (remote audio).
- Wire in an AudioPlayer and optional ApiService to fetch raw
  audio bytes from the server and play them, with event hooks
  mapped to existing lifecycle callbacks.
- Implement fallback to device TTS on server errors or empty
  responses, and ensure player lifecycle (pause/stop/dispose)
  is handled when using server engine.
- Allow engine and preferred voice to be configured before
  initialization and updated at runtime via updateSettings.

This enables selecting a server-side voice and using a remote
TTS provider while preserving compatibility with the existing
device TTS implementation.

lib/core/persistence/persistence_keys.dart

@@ -25,6 +25,9 @@ final class PreferenceKeys {
   static const String ttsSpeechRate = 'tts_speech_rate';
   static const String ttsPitch = 'tts_pitch';
   static const String ttsVolume = 'tts_volume';
+  static const String ttsEngine = 'tts_engine'; // 'device' | 'server'
+  static const String ttsServerVoiceId = 'tts_server_voice_id';
+  static const String ttsServerVoiceName = 'tts_server_voice_name';
 }
 
 final class LegacyPreferenceKeys {