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, 2 insertions, 12 deletions

diff --git a/src/commands/ai.rs b/src/commands/ai.rs
index a7a9a7e..61fb471 100644
--- a/src/commands/ai.rs
+++ b/src/commands/ai.rs
@@ -30,18 +30,8 @@ pub async fn senator(
             })
             .ok_or_else(lang::err_not_in_voice)?;
 
-        let tts_url = format!(
-            "https://translate.google.com/translate_tts?ie=UTF-8&q={}&tl=pl&client=tw-ob",
-            crate::commands::tts::urlencoding(&response)
-        );
-        let audio_data = ctx
-            .data()
-            .http_client
-            .get(&tts_url)
-            .send()
-            .await?
-            .bytes()
-            .await?;
+        let (audio_data, _) =
+            crate::tts::synthesize(&ctx.data().config, &ctx.data().http_client, &response).await?;
 
         let call = ctx
             .data()