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/queries/playlists.sql

@@ -0,0 +1,110 @@
+-- name: CreatePlaylist :one
+INSERT INTO playlists (user_id, name, description, is_public)
+VALUES ($1, $2, $3, $4)
+RETURNING *;
+
+-- name: GetPlaylist :one
+SELECT p.*, u.username AS owner_username
+FROM playlists p
+JOIN users u ON u.id = p.user_id
+WHERE p.id = $1;
+
+-- name: ListPlaylistsForUser :many
+-- Owner's playlists (any visibility) + other users' public playlists.
+-- Ordered by updated_at desc so newly-edited ones float to the top.
+SELECT p.*, u.username AS owner_username
+FROM playlists p
+JOIN users u ON u.id = p.user_id
+WHERE p.user_id = $1 OR p.is_public = true
+ORDER BY p.updated_at DESC;
+
+-- name: UpdatePlaylist :one
+-- Updates only the fields whose corresponding `updateX` flag is true.
+-- The flags let the service layer keep PATCH semantics (only-touch-what-the-caller-sent)
+-- without writing N variants.
+UPDATE playlists
+SET
+    name        = CASE WHEN sqlc.arg(update_name)::boolean        THEN sqlc.arg(name)::text        ELSE name        END,
+    description = CASE WHEN sqlc.arg(update_description)::boolean THEN sqlc.arg(description)::text ELSE description END,
+    is_public   = CASE WHEN sqlc.arg(update_is_public)::boolean   THEN sqlc.arg(is_public)::boolean ELSE is_public   END,
+    updated_at  = now()
+WHERE id = sqlc.arg(id)
+RETURNING *;
+
+-- name: UpdatePlaylistRollups :exec
+-- Set track_count + duration_sec from a fresh aggregate. Called after
+-- every mutation that touches playlist_tracks. Cheap; the table is small.
+UPDATE playlists
+SET
+    track_count  = (SELECT COUNT(*) FROM playlist_tracks pt WHERE pt.playlist_id = $1),
+    duration_sec = (SELECT COALESCE(SUM(pt.duration_sec), 0) FROM playlist_tracks pt WHERE pt.playlist_id = $1),
+    updated_at   = now()
+WHERE id = $1;
+
+-- name: SetPlaylistCover :exec
+UPDATE playlists SET cover_path = $2, updated_at = now() WHERE id = $1;
+
+-- name: DeletePlaylist :one
+-- Returns cover_path so the caller can clean up the cached collage on disk.
+DELETE FROM playlists WHERE id = $1
+RETURNING id, cover_path;
+
+-- name: ListPlaylistTracks :many
+-- Joined to tracks for the live track id (the service layer derives the
+-- stream URL from it); LEFT JOIN preserves the row when track_id is NULL
+-- (track was removed from the library). The denormalized snapshot fields
+-- on playlist_tracks remain authoritative for title/artist/album text.
+SELECT pt.*,
+       t.id           AS live_track_id,
+       albums.id      AS album_id,
+       artists.id     AS artist_id
+FROM playlist_tracks pt
+LEFT JOIN tracks  t       ON t.id       = pt.track_id
+LEFT JOIN albums           ON albums.id  = t.album_id
+LEFT JOIN artists          ON artists.id = t.artist_id
+WHERE pt.playlist_id = $1
+ORDER BY pt.position;
+
+-- name: AppendPlaylistTrack :one
+-- Inserts at the next available position. Snapshot fields are copied
+-- from the tracks/albums/artists join at insert time. tracks.duration_ms
+-- is converted to seconds for the snapshot.
+INSERT INTO playlist_tracks (playlist_id, position, track_id, title, artist_name, album_title, duration_sec)
+SELECT
+    sqlc.arg(playlist_id)::uuid,
+    COALESCE((SELECT MAX(position) + 1 FROM playlist_tracks WHERE playlist_id = sqlc.arg(playlist_id)::uuid), 0),
+    t.id,
+    t.title,
+    artists.name,
+    albums.title,
+    (t.duration_ms / 1000)::integer
+FROM tracks t
+JOIN albums  ON albums.id  = t.album_id
+JOIN artists ON artists.id = t.artist_id
+WHERE t.id = sqlc.arg(track_id)::uuid
+RETURNING *;
+
+-- name: DeletePlaylistTrack :exec
+-- Two-step: delete the row at `position`, then renumber subsequent rows
+-- to close the gap. The renumber is a single UPDATE; the service layer
+-- runs both in one transaction.
+DELETE FROM playlist_tracks
+WHERE playlist_id = $1 AND position = $2;
+
+-- name: RenumberPlaylistTracksAfter :exec
+-- Used after DeletePlaylistTrack to close the gap.
+UPDATE playlist_tracks
+SET position = position - 1
+WHERE playlist_id = $1 AND position > $2;
+
+-- name: ListAllPlaylistTracksForCollage :many
+-- First N tracks for the collage. Uses LEFT JOIN on albums for the
+-- cover_path; rows with NULL cover_path get the glyph fallback.
+SELECT pt.position,
+       albums.cover_art_path AS album_cover_path
+FROM playlist_tracks pt
+LEFT JOIN tracks t ON t.id = pt.track_id
+LEFT JOIN albums   ON albums.id = t.album_id
+WHERE pt.playlist_id = $1
+ORDER BY pt.position
+LIMIT $2;