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/taste/engagement_test.go

@@ -44,11 +44,15 @@ func TestEngagement_MonotonicNonDecreasing(t *testing.T) {
 	}
 }
 
-func TestEngagement_DegenerateParamsNoPanic(t *testing.T) {
-	// Non-increasing thresholds must collapse ramps, not divide by zero.
+func TestEngagement_DegenerateParamsStayInRange(t *testing.T) {
+	// Non-increasing thresholds must collapse ramps (no divide-by-zero) and
+	// still produce a value in [-1, 1].
 	p := EngagementParams{HardSkip: 0.5, NeutralCompletion: 0.5, FullCompletion: 0.5}
 	for _, c := range []float64{0, 0.25, 0.5, 0.75, 1} {
-		_ = Engagement(c, p)
+		got := Engagement(c, p)
+		if got < -1 || got > 1 {
+			t.Errorf("Engagement(%.2f) = %.3f out of [-1,1]", c, got)
+		}
 	}
 }