Add deleted items audit trail

[OlivierJM]

Feature Parity: Deleted Items Audit Trail

Legacy app maintains a record of deleted content for audit and potential recovery; sparked-next permanently deletes records with no tracking.

Legacy capabilities

• All deletions recorded in a deleted collection
• Stores: what was deleted, who deleted it, when, original document snapshot
• Enables audit trail for compliance
• Potential recovery path for accidentally deleted content

Acceptance criteria

• deleted_items collection in DB
• Middleware/utility: on any delete operation, write a record to deleted_items with: collection, document_id, deleted_by, deleted_at, snapshot (original document)
• GET /api/admin/deleted — list deleted items (admin only, paginated, filterable by collection)
• Admin UI: deleted items log page
• Optional: soft-delete pattern (mark as deleted rather than removing, add restore capability)

Identified via feature-parity audit against legacy Sparked (Meteor) app.