Feature: Shareable Read-Only Session Snapshots

## Problem
There is no clean way to share a specific session state with teammates without granting full dashboard access or exporting raw data manually.

## Proposal
Add **Read-Only Snapshots** that capture a session (or filtered view) and generate shareable links.

## Scope
- Snapshot creation from session detail or analytics filters
- Immutable snapshot payload (data + render metadata)
- Optional expiration + revocation controls
- Redaction options for sensitive fields before share

## UX Requirements
- One-click "Create snapshot" from key pages
- Snapshot viewer with clear read-only banner
- Watermark/timestamp indicating snapshot age

## Technical Notes
- Persist snapshots with signed IDs/tokens
- Store normalized references to source entities for traceability
- Add configurable retention policy

## Acceptance Criteria
- [ ] Users can generate shareable read-only links
- [ ] Shared views cannot mutate live data
- [ ] Snapshot expiration and revocation are enforceable
- [ ] Redaction options remove sensitive fields from output
- [ ] Audit log records snapshot creation/access/revocation