Complete owner-managed share lifecycle

[amaruki]

What to build

Let share owners manage the full lifecycle of public snapshots, including safe deletion behavior.

Acceptance criteria

• Owners can delete shares with confirmation, idempotent server behavior, and audit events.
• Public deleted snapshots return documented 410/deleted-page behavior.
• Non-owners cannot delete shares, and public pages never expose private fields after deletion.

Blocked by

#27

Slice metadata

• Type: AFK
• Sprint: Sprint 5
• User stories covered: US-14