Add playlist entity and CRUD API

[Akatenvictor]

What

Add a playlist entity and CRUD API allowing users to create, update, delete, and reorder playlists of songs.

Why

Playlists are a fundamental feature of any music platform. Users need the ability to group songs into collections for organized listening.

Scope

• In: New Playlist entity, PlaylistService, PlaylistController, playlist routes, song-to-playlist association table
• Out: Collaborative playlists, playlist sharing, auto-generated playlists, playlist import/export

Acceptance Criteria

• CRUD endpoints: POST/GET/PUT/DELETE for playlists
• POST /api/playlists/:id/songs to add a song, DELETE to remove
• GET /api/playlists/:id returns playlist with ordered songs
• Users can only modify their own playlists
• Playlists support ordering/reordering of songs
• Database migration for Playlist and PlaylistSong entities
• Unit tests for PlaylistService
• Integration tests for playlist endpoints

Technical Context

• New Playlist entity in src/entities/
• New PlaylistSong join table entity
• PlaylistService in src/services/
• PlaylistController in src/controllers/
• PlaylistRoutes in src/routes/