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, 35 insertions, 0 deletions

diff --git a/src/config.rs b/src/config.rs
index 989a060..3b92dd8 100644
--- a/src/config.rs
+++ b/src/config.rs
@@ -18,6 +18,12 @@ pub struct Config {
     pub llm_model: Option<String>,
     pub llm_api_url: String,
     pub database_path: String,
+    /// Optional path to the Piper TTS `.onnx` model (or a directory of models).
+    /// When unset, TTS falls back to Google Translate TTS.
+    pub tts_model_path: Option<String>,
+    /// Piper voice name, used to resolve the model file when `tts_model_path`
+    /// points at a directory. Ignored when it points at a `.onnx` file directly.
+    pub tts_voice: String,
 }
 
 struct EnvVar {
@@ -101,6 +107,18 @@ const ENV_VARS: &[EnvVar] = &[
         description: "SQLite database path",
     },
     EnvVar {
+        name: "TTS_MODEL_PATH",
+        required: false,
+        default: None,
+        description: "Path to a Piper TTS .onnx model (or a models directory). Unset => Google TTS fallback",
+    },
+    EnvVar {
+        name: "TTS_VOICE",
+        required: false,
+        default: Some("pl_PL-bass-high"),
+        description: "Piper TTS voice name (used when TTS_MODEL_PATH points at a directory)",
+    },
+    EnvVar {
         name: "RUST_LOG",
         required: false,
         default: Some("info"),
@@ -180,6 +198,23 @@ impl Config {
             llm_api_url: env::var("LLM_API_URL")
                 .unwrap_or_else(|_| "https://openrouter.ai/api/v1/chat/completions".into()),
             database_path: env::var("SENATOR_DB_PATH").unwrap_or_else(|_| "data/senator.db".into()),
+            tts_model_path: env::var("TTS_MODEL_PATH").ok(),
+            tts_voice: env::var("TTS_VOICE").unwrap_or_else(|_| "pl_PL-bass-high".into()),
+        }
+    }
+
+    /// Resolve the Piper model path to pass to the `piper` CLI.
+    ///
+    /// - `TTS_MODEL_PATH` set to a `.onnx` file => used verbatim.
+    /// - `TTS_MODEL_PATH` set to a directory => `{dir}/{TTS_VOICE}.onnx`.
+    /// - `TTS_MODEL_PATH` unset => `None` (caller falls back to Google TTS).
+    pub fn piper_model_path(&self) -> Option<std::path::PathBuf> {
+        let configured = self.tts_model_path.as_ref()?;
+        let path = std::path::PathBuf::from(configured);
+        if path.is_dir() {
+            Some(path.join(format!("{}.onnx", self.tts_voice)))
+        } else {
+            Some(path)
         }
     }