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/discover_test.go

@@ -163,7 +163,7 @@ func TestInterleaveBuckets_DedupsAcrossBuckets(t *testing.T) {
 }
 
 func TestInterleaveBuckets_PreservesBucketPickKind(t *testing.T) {
-	// Bucket provenance (#1270) is stamped on discoverTrack before the
+	// Bucket provenance is stamped on discoverTrack before the
 	// interleave and must survive it — including dedup, where a track in
 	// two buckets keeps the stamp of the bucket it was taken from.
 	dormant := []discoverTrack{