Add share retention cleanup and privacy audit coverage

[amaruki]

What to build

Implement retention metadata and cleanup expectations while proving share snapshots stay privacy-safe.

Acceptance criteria

• Soft-deleted snapshots have retention metadata and cleanup behavior.
• Private event, token history, and audit record retention expectations are represented.
• Tests or audit coverage verify public projections expose only allowlisted aggregate fields.

Blocked by

#28

Slice metadata

• Type: AFK
• Sprint: Sprint 5
• User stories covered: US-14, US-19