Default the English model's temperature to 0.3 (#223)

## Motivation

Human evaluations consistently prefer the English model at temperature
**0.3** over the current default **0.7**, and the change is free on
every objective axis we measured.

pocket_tts/utils/config.py

@@ -116,6 +116,7 @@ class Config(StrictModel):
     pad_with_spaces_for_short_inputs: bool = False
     remove_semicolons: bool = False
     model_recommended_frames_after_eos: int | None = None
+    default_temperature: float = 0.7
 
 
 def load_config(yaml_path: str | Path) -> Config: