Apply case-insensitive parsing to backend environment variables

Add .upper() to STT_BACKEND and TTS_BACKEND environment variable parsing
to match the case-insensitive behavior of command-line arguments.

This ensures consistency between `--stt-backend localai` and
`STT_BACKEND=localai`, both now accepting lowercase, uppercase, or
mixed case values.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: roryeckel

src/wyoming_openai/__main__.py

@@ -74,7 +74,7 @@ async def main():
         type=lambda x: OpenAIBackend[x.upper()],
         required=False,
         choices=list(OpenAIBackend),
-        default=OpenAIBackend[env_stt_backend] if env_stt_backend else None,
+        default=OpenAIBackend[env_stt_backend.upper()] if env_stt_backend else None,
         help="Backend for speech-to-text (OPENAI, SPEACHES, KOKORO_FASTAPI, LOCALAI, or None)"
     )
     parser.add_argument(
@@ -125,7 +125,7 @@ async def main():
         type=lambda x: OpenAIBackend[x.upper()],
         required=False,
         choices=list(OpenAIBackend),
-        default=OpenAIBackend[env_tts_backend] if env_tts_backend else None,
+        default=OpenAIBackend[env_tts_backend.upper()] if env_tts_backend else None,
         help="Backend for text-to-speech (OPENAI, SPEACHES, KOKORO_FASTAPI, LOCALAI, or None)"
     )
     parser.add_argument(