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/dbq/models.go

@@ -335,6 +335,30 @@ type PlaySession struct {
 	ClientID    *string
 }
 
+type Playlist struct {
+	ID          pgtype.UUID
+	UserID      pgtype.UUID
+	Name        string
+	Description string
+	IsPublic    bool
+	CoverPath   *string
+	TrackCount  int32
+	DurationSec int32
+	CreatedAt   pgtype.Timestamptz
+	UpdatedAt   pgtype.Timestamptz
+}
+
+type PlaylistTrack struct {
+	PlaylistID  pgtype.UUID
+	Position    int32
+	TrackID     pgtype.UUID
+	Title       string
+	ArtistName  string
+	AlbumTitle  string
+	DurationSec int32
+	AddedAt     pgtype.Timestamptz
+}
+
 type ScrobbleQueue struct {
 	ID            pgtype.UUID
 	UserID        pgtype.UUID