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/manual_test.go

@@ -32,8 +32,7 @@ func TestParseMusicBrainzRef(t *testing.T) {
 }
 
 // A person names the MusicBrainz album an unmatched album is, and it goes
-// on the list as held; or leaves it off, and matching never tries it again
-// (#5579).
+// on the list as held; or leaves it off, and matching never tries it again.
 func TestMatchAlbumTo_ByLinkOrLeaveItOff_Integration(t *testing.T) {
 	pool := newPool(t)
 	ctx := context.Background()
@@ -120,7 +119,7 @@ func TestMatchAlbumTo_ByLinkOrLeaveItOff_Integration(t *testing.T) {
 
 // A re-tag files a library track under a new album row. That album is the
 // library's, not one dropped in by hand, so the listing matches it; the
-// old row, its tracks gone, is no longer unmatched (#5579).
+// old row, its tracks gone, is no longer unmatched.
 func TestListLibrary_MatchesARetaggedAlbum_Integration(t *testing.T) {
 	pool := newPool(t)
 	ctx := context.Background()