feat(avatar): add private vision chat support

digital-avatar-app/backend/services/chat_attachment_service.py

@@ -0,0 +1,20 @@
+from datetime import datetime
+
+from sqlalchemy.orm import Session
+
+from models import ChatAttachment
+
+
+def purge_expired_chat_attachments(
+    db: Session,
+    *,
+    now: datetime | None = None,
+) -> int:
+    """Remove expired derived image data; raw image bytes are never persisted."""
+    count = db.query(ChatAttachment).filter(
+        ChatAttachment.expires_at < (now or datetime.utcnow())
+    ).delete(synchronize_session=False)
+    if count:
+        db.commit()
+        db.expire_all()
+    return count