Add home page carousel/slide management

[OlivierJM]

Feature Parity: Slide/Carousel Management

Legacy app allows admins to manage carousel/slider content on the landing page; sparked-next has a static hero section with no admin-managed slides.

Legacy capabilities

• Create slides with image upload
• Edit slide images and content
• Delete slides
• Slides displayed as image carousel on landing/home page
• Admin-controlled from dashboard

Acceptance criteria

• slides collection in DB
• POST /api/admin/slides — create slide with image upload
• PUT /api/admin/slides/:id — edit slide
• DELETE /api/admin/slides/:id — delete slide
• GET /api/slides — list slides (public)
• Admin UI: slide management page
• Landing page updated to render slides as carousel (replace or supplement static hero)
• Images uploaded to S3

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