Add search index management for admins

[OlivierJM]

Feature Parity: Search Data Management

Legacy app gives admins tools to manage and optimize the search index; sparked-next has no search administration.

Legacy capabilities

• Admin can view and remove stale/outdated search entries
• Bulk update search index data
• Rebuild search index for specific content types
• Search data stored in dedicated search collection with category metadata

Acceptance criteria

• GET /api/admin/search — list search index entries (paginated)
• DELETE /api/admin/search/:id — remove stale search entry
• POST /api/admin/search/rebuild — rebuild search index from current content
• Admin UI: search management page (list entries, bulk delete, rebuild button)
• Search entries auto-updated when media content is created/edited/deleted

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