feat: add playlist entity and CRUD API (Closes #77)

[achiever2110]

Summary

Adds a Playlist entity and full CRUD API allowing users to create, update, delete, and reorder playlists of songs.

Closes #77

Changes

• Entities: Playlist, PlaylistSong (join table with ordering)
• Migration: 1753200000004-AddPlaylistEntities creates playlists and playlist_songs tables
• Service: PlaylistService — CRUD, song add/remove, reordering, ownership enforcement
• Controller/Routes: PlaylistController + routes under /api/playlists
  • POST /api/playlists — create
  • GET /api/playlists — list mine (paginated)
  • GET /api/playlists/:id — playlist with songs ordered by position
  • PUT /api/playlists/:id — update metadata
  • DELETE /api/playlists/:id — delete
  • POST /api/playlists/:id/songs — add song
  • DELETE /api/playlists/:id/songs/:songId — remove song
  • PUT /api/playlists/:id/reorder — reorder songs
• Users can only modify their own playlists; private playlists are owner-only

Tests

• PlaylistService unit tests (18) and PlaylistController tests (12), all passing

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

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