feat: enhance logging for character updates and voice cache management

qwen3-tts-backend/core/audiobook_service.py

@@ -1474,6 +1474,12 @@ async def generate_character_preview(project_id: int, char_id: int, user: User,
     backend = await TTSServiceFactory.get_backend()
 
     try:
+        if force_recreate and design.voice_cache_id:
+            design.voice_cache_id = None
+            db.commit()
+            db.refresh(design)
+            logger.info(f"Cleared voice_cache_id for char {char_id} (force_recreate)")
+
         if not design.voice_cache_id:
             logger.info(f"Local voice cache missing for char {char_id}. Bootstrapping now...")
             from core.model_manager import ModelManager