feat: add personal playlists and shareable track links

EOF

Co-authored-by: Cursor <cursoragent@cursor.com>

services/api/migrations/00013_playlists.sql

@@ -0,0 +1,23 @@
+-- +goose Up
+CREATE TABLE playlists (
+  id uuid PRIMARY KEY,
+  user_id uuid NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  name text NOT NULL CHECK (char_length(name) BETWEEN 1 AND 120),
+  description text NOT NULL DEFAULT '' CHECK (char_length(description) <= 2000),
+  revision integer NOT NULL DEFAULT 1,
+  created_at timestamptz NOT NULL DEFAULT now(),
+  updated_at timestamptz NOT NULL DEFAULT now()
+);
+CREATE INDEX playlists_owner ON playlists(user_id, updated_at DESC, id);
+CREATE TABLE playlist_tracks (
+  playlist_id uuid NOT NULL REFERENCES playlists(id) ON DELETE CASCADE,
+  track_id uuid NOT NULL REFERENCES tracks(id) ON DELETE CASCADE,
+  position integer NOT NULL CHECK (position >= 0),
+  PRIMARY KEY (playlist_id, track_id),
+  UNIQUE (playlist_id, position)
+);
+CREATE INDEX playlist_tracks_track ON playlist_tracks(track_id);
+
+-- +goose Down
+DROP TABLE playlist_tracks;
+DROP TABLE playlists;