docs: wanted, dbtest, recommendation and playlists comments say what the code does (#6277)

The same rewrite as d191a282, for four packages whose comments carried the
most history: tuning stories, quotes from the operator, single-install
measurements in library_scale.go, and 'used to' accounts of bugs. The
constraint each story protected stays as a present-tense reason (why tier 3
is reduced not removed, why the randomised arms take an order seed, why the
seed artist is eligible in Songs-like). Comments naming system_cron.go and
config.RecommendationConfig, neither of which holds what they claimed, now
name the scheduler and internal/recsettings.

Checked: non-comment tokens identical to HEAD; gofmt clean.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/wanted/upgrade_test.go

@@ -99,7 +99,7 @@ func TestCheckUpgrades_AHeldAlbumBelowTheCutoffIsWantedAgain_Integration(t *test
 		t.Fatalf("due for search: %+v, %v", due, err)
 	}
 
-	// Upgrades switched off: no longer wanted again.
+	// Upgrades switched off: the album is not wanted again.
 	if _, err := pool.Exec(ctx, `UPDATE acquisition_settings SET upgrade_until = ''`); err != nil {
 		t.Fatal(err)
 	}
@@ -138,9 +138,8 @@ func TestCheckUpgrades_ARipIsReplacedWhateverTheSetting_Integration(t *testing.T
 }
 
 // A missing track with no disc number is on disc 1, so a present 1/2 stands in
-// for a missing "2". Air's Love 2 stayed wanted over one such row and was
-// downloaded again for nothing (2026-10-20). A missing track no present track
-// covers still counts.
+// for a missing "2", and the album is not downloaded again for nothing. A
+// missing track no present track covers still counts.
 func TestWantHoldings_AMissingTrackWithNoDiscIsOnDiscOne_Integration(t *testing.T) {
 	pool := newPool(t)
 	ctx := context.Background()