feat(server/m7-352): BuildSystemPlaylists with atomic replace + concurrency guard

Implements T5 of #352 slice 2: adds CreateSystemPlaylist sqlc query,
BuildSystemPlaylists function (For-You + Songs-like mixes, tx atomic
replace, in_flight guard, tieBreakHash determinism), and 7 integration
tests covering activity, quarantine exclusion, atomic replace, concurrency,
daily nonce stability, ListActiveUsers, and stale-in-flight recovery.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/db/dbq/system_playlists.sql.go

@@ -23,6 +23,52 @@ func (q *Queries) ClearStaleSystemPlaylistInFlight(ctx context.Context) error {
 	return err
 }
 
+const createSystemPlaylist = `-- name: CreateSystemPlaylist :one
+INSERT INTO playlists (
+    user_id, name, description, is_public,
+    kind, system_variant, seed_artist_id, cover_path
+) VALUES ($1, $2, '', false, 'system', $3, $4, $5)
+RETURNING id, user_id, name, description, is_public, cover_path, track_count, duration_sec, created_at, updated_at, kind, system_variant, seed_artist_id
+`
+
+type CreateSystemPlaylistParams struct {
+	UserID        pgtype.UUID
+	Name          string
+	SystemVariant *string
+	SeedArtistID  pgtype.UUID
+	CoverPath     *string
+}
+
+// Inserts a system-generated playlist. Used by BuildSystemPlaylists.
+// For 'for_you' variant, pass seed_artist_id as NULL (zero-value pgtype.UUID
+// with Valid=false).
+func (q *Queries) CreateSystemPlaylist(ctx context.Context, arg CreateSystemPlaylistParams) (Playlist, error) {
+	row := q.db.QueryRow(ctx, createSystemPlaylist,
+		arg.UserID,
+		arg.Name,
+		arg.SystemVariant,
+		arg.SeedArtistID,
+		arg.CoverPath,
+	)
+	var i Playlist
+	err := row.Scan(
+		&i.ID,
+		&i.UserID,
+		&i.Name,
+		&i.Description,
+		&i.IsPublic,
+		&i.CoverPath,
+		&i.TrackCount,
+		&i.DurationSec,
+		&i.CreatedAt,
+		&i.UpdatedAt,
+		&i.Kind,
+		&i.SystemVariant,
+		&i.SeedArtistID,
+	)
+	return i, err
+}
+
 const deleteSystemPlaylistsForUser = `-- name: DeleteSystemPlaylistsForUser :exec
 DELETE FROM playlists WHERE user_id = $1 AND kind = 'system'
 `