fix(taste): drop unused now param + tighten test (golangci-lint revive)

golangci-lint v2 (CI-only; local is v1) flagged two unused-parameter issues:
- BuildTasteProfile's `now` was genuinely dead — decay/windowing are computed
  DB-side via now(), so no Go-side timestamp is threaded. Removed it (a
  phase-3 context model that needs a pinned reference time would re-add it);
  updated the scheduler call site.
- the degenerate-params engagement test ignored t; reworked it to assert the
  result stays in [-1,1], which also strengthens the test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/playlists/scheduler.go

@@ -227,7 +227,7 @@ func (s *Scheduler) runStartupCatchUp(ctx context.Context, users []dbq.ListActiv
 // build can read a fresh profile (phase-2 consumption); both steps are
 // best-effort and a failure in one is logged without blocking the other.
 func (s *Scheduler) rebuildUserDaily(ctx context.Context, userID pgtype.UUID, now time.Time) {
-	if err := taste.BuildTasteProfile(ctx, s.pool, s.logger, userID, now, taste.DefaultConfig()); err != nil {
+	if err := taste.BuildTasteProfile(ctx, s.pool, s.logger, userID, taste.DefaultConfig()); err != nil {
 		s.logger.Warn("scheduler: taste profile rebuild failed",
 			"user_id", uuidStringPL(userID), "err", err)
 	}