-- M7 #352 slice 2: system-generated playlists.
-- Adds kind/system_variant/seed_artist_id columns to playlists and a
-- per-user run-tracking table. Backfills existing slice-1 playlists
-- to kind='user'.

-- Discriminator on existing playlists table.
ALTER TABLE playlists
    ADD COLUMN kind            text NOT NULL DEFAULT 'user',
    ADD COLUMN system_variant  text,
    ADD COLUMN seed_artist_id  uuid REFERENCES artists(id) ON DELETE SET NULL;

-- Belt-and-braces backfill in case the DEFAULT didn't apply on some
-- runtime branches; a no-op when the column metadata already filled.
UPDATE playlists SET kind = 'user' WHERE kind IS NULL OR kind = '';

-- Constraint: kind/system_variant pair is consistent.
ALTER TABLE playlists
    ADD CONSTRAINT playlists_kind_variant_consistent
    CHECK (
        (kind = 'user'   AND system_variant IS NULL)
        OR
        (kind = 'system' AND system_variant IN ('for_you', 'songs_like_artist'))
    );

-- Constraint: 'songs_like_artist' has a seed artist; nothing else does.
ALTER TABLE playlists
    ADD CONSTRAINT playlists_seed_consistent
    CHECK (
        (system_variant IS NULL              AND seed_artist_id IS NULL)
        OR
        (system_variant = 'for_you'          AND seed_artist_id IS NULL)
        OR
        (system_variant = 'songs_like_artist' AND seed_artist_id IS NOT NULL)
    );

-- Filter index for kind=user vs kind=system queries on the user's row.
CREATE INDEX playlists_user_kind_idx ON playlists (user_id, kind);

-- Per-user run tracking — one row per user.
CREATE TABLE system_playlist_runs (
    user_id        uuid PRIMARY KEY REFERENCES users(id) ON DELETE CASCADE,
    last_run_at    timestamptz,
    last_run_date  date,
    in_flight      boolean NOT NULL DEFAULT false,
    last_error     text
);