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/wanted.go

@@ -1,10 +1,9 @@
-// Package wanted is what Minstrel is looking for (M502 #5520): followed
-// artists, each on a release-type profile, and wanted release groups. A want
-// comes from an approved request, a followed artist's new release, or an
-// album whose files went missing; it is held once the library has a full
-// copy. The library's own albums were put on the list as held once (#5567),
-// so the list is the record of what the library keeps. Searching for open
-// wants is #5521's.
+// Package wanted is what Minstrel is looking for: followed artists, each on
+// a release-type profile, and wanted release groups. A want comes from an
+// approved request, a followed artist's new release, or an album whose files
+// went missing; it is held once the library has a full copy. The library's
+// own albums are put on the list as held once, so the list is the record of
+// what the library keeps. Searching for open wants is the Searcher's.
 package wanted
 
 import (
@@ -47,11 +46,11 @@ type Service struct {
 	logger *slog.Logger
 	now    func() time.Time
 	// releaseGroup names the group of a MusicBrainz release id, to move a
-	// want recorded under a release id (#5241) to its group. A field so tests
+	// want recorded under a release id to its group. A field so tests
 	// stand in for MusicBrainz.
 	releaseGroup func(ctx context.Context, releaseMBID string) (string, error)
 	// upgradesChecked is when held albums were last judged for an upgrade
-	// (#5524); the worker alone touches it.
+	// the worker alone touches it.
 	upgradesChecked time.Time
 }
 
@@ -196,9 +195,9 @@ type PassResult struct {
 	Baselined int // of those, first reads
 	Wanted    int // wants recorded from follows
 	Held      int // wants the library now answers
-	Listed    int // library albums put on the list as held (#5567)
+	Listed    int // library albums put on the list as held
 	// Upgrades: held albums wanted for a better copy, when the hourly check
-	// ran this pass (#5524).
+	// ran this pass.
 	Upgrades int
 }
 
@@ -211,7 +210,7 @@ func (s *Service) Pass(ctx context.Context) (PassResult, error) {
 	if ctx.Err() != nil {
 		return res, ctx.Err()
 	}
-	// The one-time listing of the library (#5567); a failure waits for the
+	// The one-time listing of the library; a failure waits for the
 	// next pass and does not hold back settling.
 	if err := s.listLibrary(ctx, &res); err != nil && ctx.Err() == nil {
 		s.logger.Warn("wanted: list the library", "err", err)
@@ -321,9 +320,9 @@ func (s *Service) applyFollow(ctx context.Context, q *dbq.Queries, f dbq.Follow)
 	return wanted, q.MarkFollowChecked(ctx, f.ArtistMbid)
 }
 
-// dropRefusedVersion takes off the list a group the follow wanted before
-// Passes read the version its title names (#5573), while it is still only
-// wanted: one the operator holds, or wanted for another reason, stays.
+// dropRefusedVersion takes off the list a group a follow wants that Passes
+// now refuses for the version its title names, while it is still only
+// wanted: one the library holds, or wanted for another reason, stays.
 func (s *Service) dropRefusedVersion(ctx context.Context, q *dbq.Queries, group, title string) error {
 	v, ok := release.VersionInTitle(title)
 	if !ok {
@@ -464,7 +463,7 @@ func Shortest(releases []dbq.MbRelease) int {
 }
 
 // moveToGroup handles a want MusicBrainz has no release group under: one
-// recorded under a release id before #5241. It is moved to the release's
+// recorded under a release id. It is moved to the release's
 // group, with the requests that name it; when the group is already wanted,
 // this copy is dropped.
 func (s *Service) moveToGroup(ctx context.Context, w dbq.Want) (bool, error) {
@@ -509,9 +508,8 @@ func (s *Service) moveToGroup(ctx context.Context, w dbq.Want) (bool, error) {
 //     missing track is left without a track on disk in its place;
 //   - otherwise an album of the group with at least as many tracks on disk as
 //     the shortest official edition;
-//   - and when the catalogue cannot say how long the group is, the test from
-//     #5263: a track arrived since the want, or nothing missing before it is
-//     still missing.
+//   - and when the catalogue cannot say how long the group is: a track
+//     arrived since the want, or nothing missing before it is still missing.
 //
 // Only albums with a track on disk count.
 func (s *Service) answeredBy(ctx context.Context, q *dbq.Queries, w dbq.Want, shortest int) (pgtype.UUID, bool, error) {