Set Story Sharing

Enable/disable link sharing for a story WITHOUT changing feed visibility (owner only) — the "private-but-linkable" state. shared=true -> mint a share_token (reuse existing); is_public unchanged. shared=false -> discard the token (kills all ?token= links) AND set is_public=false (a story can't stay in the feed with no token). This is the "turn off sharing" action. Body: {"shared": bool}