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, 7 insertions, 0 deletions

diff --git a/src/lang.rs b/src/lang.rs
index 05e82fd..aad0413 100644
--- a/src/lang.rs
+++ b/src/lang.rs
@@ -114,6 +114,13 @@ pub fn fmt_voice_join_failed(e: &dyn std::fmt::Display) -> String {
 pub const L_TTS_SAVED: &str = "TTS zapisany";
 pub const L_TTS_PLAYING: &str = "Grom TTS na kanale głosowym";
 
+pub fn fmt_tts_piper_launch(e: &str) -> String {
+    format!("Piper sie nie uruchomił w czas: {e}")
+}
+pub fn fmt_tts_piper_failed(e: &str) -> String {
+    format!("Piper TTS sie nie powiodło: {e}")
+}
+
 // ── Trigger command ───────────────────────────────────────────────────
 
 pub const L_TRIGGER_UPDATED: &str = "Trigger zaktualizowany";