feat(db/m7-352): system playlists schema (kind, variant, seed_artist_id, runs table)

internal/db/migrations/0015_system_playlists.down.sql

@@ -0,0 +1,10 @@
+DROP TABLE IF EXISTS system_playlist_runs;
+DROP INDEX IF EXISTS playlists_user_kind_idx;
+ALTER TABLE playlists
+    DROP CONSTRAINT IF EXISTS playlists_seed_consistent;
+ALTER TABLE playlists
+    DROP CONSTRAINT IF EXISTS playlists_kind_variant_consistent;
+ALTER TABLE playlists
+    DROP COLUMN IF EXISTS seed_artist_id,
+    DROP COLUMN IF EXISTS system_variant,
+    DROP COLUMN IF EXISTS kind;

internal/db/migrations/0015_system_playlists.up.sql

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+-- 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
+);