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/match.go

@@ -16,13 +16,11 @@ import (
 	"git.fabledsword.com/bvandeusen/minstrel/internal/words"
 )
 
-// Matching a library album to its MusicBrainz release group (#5569).
+// Matching a library album to its MusicBrainz release group.
 //
-// Everything in the library came from a service that matched it against
-// MusicBrainz (operator, 2026-10-10: "they should all exist in music brainz
-// we need to use their meta data or a search system to link them up"), so
-// every album should have a group. The ids its tags carry are taken as
-// they are. Without them, the groups called what the album is called, from
+// Music from any mainstream source was matched against MusicBrainz on its
+// way in, so nearly every album has a group. The ids its tags carry are
+// taken as they are. Without them, the groups called what the album is called, from
 // the artist's discography and then from a search, are checked against the
 // album's own tracks, and the one its tracks agree with is taken.