Add artist follow/unfollow system with feed

[Akatenvictor]

What

Add an artist follow/unfollow system with follower counts and a feed endpoint showing songs from followed artists.

Why

Social features drive engagement. Users need to follow their favorite artists to stay updated on new releases, and artists benefit from follower metrics.

Scope

• In: Follow entity, follow/unfollow endpoints, follower/following counts, feed endpoint
• Out: Activity feed with timeline, suggested follows, mutual follows

Acceptance Criteria

• POST /api/artists/:id/follow follows an artist
• DELETE /api/artists/:id/follow unfollows
• GET /api/artists/:id/followers returns follower list
• GET /api/artists/:id/following returns who the artist follows
• GET /api/feed returns recent songs from followed artists
• Follower/following counts exposed on artist profile response
• Prevent duplicate follows (idempotent)
• Database migration for UserFollow join entity

Technical Context

• New UserFollow entity in src/entities/
• ArtistProfileService in src/services/ArtistProfileService for profile logic
• ArtistProfileController for follow endpoints
• Artist routes in src/routes/artistRoutes
• Feed generation in SongService