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/profile.go

@@ -11,7 +11,6 @@ import (
 	"log/slog"
 	"sort"
 	"strings"
-	"time"
 
 	"github.com/jackc/pgx/v5/pgtype"
 	"github.com/jackc/pgx/v5/pgxpool"
@@ -81,9 +80,13 @@ func DefaultConfig() Config {
 // Self-contained transaction for the replace. Returns an error only on a
 // fatal DB failure; the daily caller logs and proceeds to playlist building
 // regardless (best-effort).
+//
+// The decay reference time is the database clock (age is computed in SQL via
+// now()), so no Go-side timestamp is threaded here; a phase-3 context model
+// that needs a pinned reference time would add one.
 func BuildTasteProfile(
 	ctx context.Context, pool *pgxpool.Pool, logger *slog.Logger,
-	userID pgtype.UUID, now time.Time, cfg Config,
+	userID pgtype.UUID, cfg Config,
 ) error {
 	q := dbq.New(pool)