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/playlists/foryou_test.go

@@ -246,7 +246,7 @@ func TestPickTopN_DiversityCap(t *testing.T) {
 
 func TestPickHeadAndTail_MarksPickKinds(t *testing.T) {
 	// 100-deep pool with headN=20, tailN=5: the 20 head entries are the
-	// taste picks, the 5 tail entries the freshness injection (#1249).
+	// taste picks, the 5 tail entries the freshness injection.
 	in := make([]recommendation.Candidate, 0, 100)
 	for i := 0; i < 100; i++ {
 		in = append(in, makeCand(i+1, i+1, i+1, float64(100-i)))
@@ -323,7 +323,7 @@ func TestHeadQuotas(t *testing.T) {
 }
 
 func TestPickQuotaHead_BlendsSeeds(t *testing.T) {
-	// Seed 0's neighborhood out-scores everything (#1269): without
+	// Seed 0's neighborhood out-scores everything: without
 	// quotas the head would be all seed-0. With headN=10 and quotas
 	// [5,3,2], the walk takes the best 5 from seed 0, then seeds 1 and
 	// 2 get their guaranteed slots from deeper ranks.
@@ -385,7 +385,7 @@ func TestPickQuotaHead_ThinSeedSpillsBestFirst(t *testing.T) {
 }
 
 func TestPickWeightedTail_PrefersEarlyRanks(t *testing.T) {
-	// The fresh tail's sample is rank-weighted (#1269): across many
+	// The fresh tail's sample is rank-weighted: across many
 	// days, the mean selected rank must sit well below the uniform
 	// draw's ~99.5 for a 200-deep pool.
 	pool := make([]recommendation.Candidate, 0, 200)