Remove auto support for whisper_device; only cpu and cuda allowed

- Validate whisper_device in Settings and _get_local_model
- Reject 'auto' with clear ValueError/ValidationError
- Update docs in config, .env.example, README
- Add tests for invalid device and valid cpu/cuda

Co-authored-by: Ali Khokhar <alishahryar2@gmail.com>

config/settings.py

@@ -80,7 +80,7 @@ class Settings(BaseSettings):
     hf_token: str = Field(default="", validation_alias="HF_TOKEN")
     # Model size: "tiny" | "base" | "small" | "medium" | "large-v2" | "large-v3" | "large-v3-turbo"
     whisper_model: str = Field(default="base", validation_alias="WHISPER_MODEL")
-    # Device: "cpu" | "cuda" | "auto" (auto = try cuda, fail fast; no fallback)
+    # Device: "cpu" | "cuda"
     whisper_device: str = Field(default="cpu", validation_alias="WHISPER_DEVICE")
 
     # ==================== Bot Wrapper Config ====================
@@ -115,6 +115,13 @@ class Settings(BaseSettings):
             return None
         return v
 
+    @field_validator("whisper_device")
+    @classmethod
+    def validate_whisper_device(cls, v: str) -> str:
+        if v not in ("cpu", "cuda"):
+            raise ValueError(f"whisper_device must be 'cpu' or 'cuda', got {v!r}")
+        return v
+
     model_config = SettingsConfigDict(
         env_file=".env",
         env_file_encoding="utf-8",