feat(db): playlists schema for M7 #352 slice 1

Migration 0014 adds playlists + playlist_tracks. track_id is nullable
with ON DELETE SET NULL — tracks can be removed from the library
without silently dropping playlist entries; the denormalized snapshot
(title/artist/album/duration) keeps the row legible afterwards. UI
renders such rows greyed-out.

Indexes: playlists by (user_id, updated_at DESC) and a partial public
index for cross-user discovery; playlist_tracks partial index on
track_id to support the FK SET NULL lookup.

Queries provide CRUD + rollup recompute (track_count, duration_sec)
+ append/remove primitives. Reorder is service-layer orchestrated via
raw tx.Exec; no SQL primitive needed.

internal/db/migrations/0014_playlists.down.sql

@@ -0,0 +1,2 @@
+DROP TABLE IF EXISTS playlist_tracks;
+DROP TABLE IF EXISTS playlists;

internal/db/migrations/0014_playlists.up.sql

@@ -0,0 +1,39 @@
+-- M7 #352 slice 1: playlists CRUD foundation.
+-- See docs/superpowers/specs/2026-05-03-m7-playlists-crud-design.md.
+
+CREATE TABLE playlists (
+    id           uuid        PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id      uuid        NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    name         text        NOT NULL,
+    description  text        NOT NULL DEFAULT '',
+    is_public    boolean     NOT NULL DEFAULT false,
+    cover_path   text,
+    track_count  integer     NOT NULL DEFAULT 0,
+    duration_sec integer     NOT NULL DEFAULT 0,
+    created_at   timestamptz NOT NULL DEFAULT now(),
+    updated_at   timestamptz NOT NULL DEFAULT now()
+);
+
+CREATE INDEX playlists_user_idx   ON playlists (user_id, updated_at DESC);
+CREATE INDEX playlists_public_idx ON playlists (is_public, updated_at DESC) WHERE is_public;
+
+-- track_id is nullable + ON DELETE SET NULL so deleting a track from
+-- the library doesn't silently drop entries from operators' playlists.
+-- Denormalized title/artist/album/duration carry a snapshot so the row
+-- is still legible after the upstream track row is gone.
+CREATE TABLE playlist_tracks (
+    playlist_id  uuid        NOT NULL REFERENCES playlists(id) ON DELETE CASCADE,
+    position     integer     NOT NULL,
+    track_id     uuid        REFERENCES tracks(id) ON DELETE SET NULL,
+    title        text        NOT NULL,
+    artist_name  text        NOT NULL,
+    album_title  text        NOT NULL,
+    duration_sec integer     NOT NULL,
+    added_at     timestamptz NOT NULL DEFAULT now(),
+    PRIMARY KEY (playlist_id, position)
+);
+
+-- Partial index supports the FK lookup that fires when a track is
+-- deleted (ON DELETE SET NULL) — without it, deleting a track triggers
+-- a full scan of playlist_tracks.
+CREATE INDEX playlist_tracks_track_idx ON playlist_tracks (track_id) WHERE track_id IS NOT NULL;