Implement album cover art upload and retrieval

[Akatenvictor]

What

Implement an album cover art upload and retrieval endpoint that stores cover images via Pinata and serves optimized thumbnails.

Why

Albums and songs currently lack a standardized way to display cover artwork. Cover art is essential for user experience and marketplace listing quality.

Scope

• In: Cover art upload via Pinata, thumbnail generation, entity association, retrieval endpoints
• Out: Cover art editing/replacement, AI-generated artwork, aspect ratio validation

Acceptance Criteria

• POST /api/songs/:id/cover-art uploads and associates cover art
• Cover art stored in Pinata via existing PinataService
• Thumbnail variants generated: 150px, 300px, 600px
• GET /api/songs/:id/cover-art returns cover art URLs including thumbnails
• File size limit: 10MB max
• Supported formats: JPEG, PNG, WebP
• Cover art URL stored on Song entity

Technical Context

• Extend existing PinataService for cover art pinning
• Song entity needs coverArtIpfsHash field
• Upload middleware in src/middlewares/upload for file handling
• ImageFile validator in src/validators/ for format validation
• SongProcessorWorker could handle thumbnail generation