feat: local TTS via piper subprocess with Google fallback

- New src/tts.rs: centralize TTS in synthesize(config, http_client, text)
  Piper subprocess primary, Google Translate fallback on error/missing
- TTS_MODEL_PATH/TTS_VOICE env config (defaults to pl_PL-bass-high)
- Fix Unknown interaction errors: spawn play_tts_in_channel in background
  so voice-state TTS no longer blocks the shard event loop (3s Discord limit)
- Best-effort confirmation replies in /tts to prevent error cascade
- Custom on_error handler: graceful interaction-expired error handling
- Comprehensive tracing logging (engine, model, duration, stderr)

1 files changed, 6 insertions, 2 deletions

diff --git a/.env.example b/.env.example
index fe76c05..01e24b3 100644
--- a/.env.example
+++ b/.env.example
@@ -3,13 +3,17 @@ TOKEN=
 
 # Optional - Bot Configuration
 STATUS_CHANNEL_ID=
-ASSISTANT_KEYWORD=jarvis  # Keyword to trigger AI assistant
 
 # Optional - Copyparty File Server (for music /search commands)
 COPYPARTY_URL=http://localhost:8080
 COPYPARTY_URL_UPLOAD=http://localhost:8080/uploads
 COPYPARTY_PASSWORD=
 
-# Optional - AI commands
+# Optional - AI functionality
+ASSISTANT_KEYWORD=jarvis  # Keyword to trigger AI assistant
 OPENROUTER_API_KEY=
 OPENROUTER_MODEL=
+
+# Optional - Local TTS
+TTS_MODEL_PATH=/path/to/models          # or a full path to pl_PL-bass-high.onnx
+TTS_VOICE=pl_PL-bass-high