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/seed_selection_test.go

@@ -48,9 +48,9 @@ func TestPickSeedArtistsFromRows_Empty(t *testing.T) {
 }
 
 func TestPickKindForSeedTier(t *testing.T) {
-	// The seed query's fallback tiers map onto the rule-#131 ladder:
-	// fresh 7-day engagement is the exact desire, everything past the
-	// 30-day step-back collapses into the far tier (#1255).
+	// The seed query's fallback tiers map onto the tier ladder: fresh
+	// 7-day engagement is the exact desire, everything past the 30-day
+	// step-back collapses into the far tier.
 	cases := []struct {
 		tier int32
 		want string
@@ -99,7 +99,7 @@ func TestUserIDHash_DifferentUserChangesHash(t *testing.T) {
 
 // pickDailySeeds shuffles the candidate pool daily-deterministically
 // and takes up to n — For-You uses n=forYouSeedCount for its
-// multi-seed blend (#1269), Songs-like uses n=songsLikeSeedCount via
+// multi-seed blend, Songs-like uses n=songsLikeSeedCount via
 // the pickSeedArtistsForDay wrapper. Verifies determinism within a day,
 // variation across days, and graceful degradation on small pools.