Add genre-based song browsing endpoint

[Akatenvictor]

What

Add a genre-based browsing endpoint that returns songs filtered by genre with pagination and sorting options.

Why

Discoverability is critical for an audio platform. Users need to browse content by genre to find music they enjoy, and the current API provides no genre-based filtering.

Scope

• In: New GET /api/genres/:id/songs endpoint, genre listing endpoint, pagination support
• Out: Genre recommendation engine, sub-genre taxonomy, mood-based tagging

Acceptance Criteria

• GET /api/genres returns all available genres with song counts
• GET /api/genres/:id/songs returns songs in that genre with pagination
• Supports sort by: newest, most played, alphabetical
• Pagination via query params: page, limit (default 20, max 100)
• Response includes total count for client-side pagination
• Genre entity properly linked to Song entity via foreign key

Technical Context

• Genre entity exists in src/entities/Genre
• Song entity should have genre_id foreign key
• SongService can be extended with genre filtering
• SongController needs the genre-based methods
• SongRoutes needs the new routes