feat: Add status and error fields to TTS model and implement background processing for TTS generations

app/models/tts.py

@@ -20,6 +20,8 @@ class TTS(SQLModel, table=True):
         sa_column=Column(JSON),
         description="Provider-specific options used"
     )
+    status: str = Field(default="pending", description="Processing status")
+    error: str | None = Field(default=None, description="Error message if failed")
     sound_id: int | None = Field(foreign_key="sound.id", description="Associated sound ID")
     user_id: int = Field(foreign_key="user.id", description="User who created the TTS")
     created_at: datetime = Field(default_factory=datetime.utcnow)