feat: add voice studio prototype flow

backend/app/core/config.py

@@ -70,6 +70,18 @@ class Settings(BaseSettings):
         "storage/voice_sessions",
         description="Directory for persisted voice co-creation session assets",
     )
+    voice_transcription_mode: str = Field(
+        "demo",
+        description="Voice transcription mode: demo, openai, or disabled",
+    )
+    voice_transcription_model: str = Field(
+        "gpt-4o-mini-transcribe",
+        description="Model used when voice transcription mode is OpenAI-backed",
+    )
+    voice_transcription_language: str = Field(
+        "zh",
+        description="Preferred language hint for voice transcription",
+    )
     story_audio_cache_ttl_days: int = Field(
         30,
         description="TTL in days before cached story audio is pruned",