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/library.go

@@ -12,7 +12,7 @@ import (
 )
 
 // ReasonLibrary: on the list because the library held it when the list
-// began (#5567).
+// began.
 const ReasonLibrary = "library"
 
 // libraryMatchBudget bounds the time one pass spends matching albums to
@@ -33,17 +33,17 @@ const libraryMatchBatch = 25
 //	4  an album holding every track of an edition, and more, is that group
 const libraryMatcher = 4
 
-// listLibrary puts the library's albums on the wanted list as held, once
-// (operator, 2026-10-10: "make a record saying that the import has
-// happened so later changes like unwanting hold and dont auto import from
-// the file directory"). It runs a step each pass until done:
+// listLibrary puts the library's albums on the wanted list as held, once,
+// so that from then on the list is the record of what the library keeps: an
+// album taken off it stays off, and files dropped in by hand are not listed.
+// It runs a step each pass until done:
 //
 //   - the first step stamps when the listing began, which is its cutoff:
 //     an album the library gains afterwards, dropped in by hand, is never
 //     listed;
 //   - every album whose tags name its release group and artist goes on the
 //     list at once;
-//   - every other album is matched to MusicBrainz (#5569), within a time
+//   - every other album is matched to MusicBrainz, within a time
 //     budget a pass, and listed as it is matched;
 //   - once none is left to match, the listing is stamped done, with a count
 //     of the albums no match was found for.
@@ -51,8 +51,8 @@ const libraryMatcher = 4
 // Each album decided is recorded in library_album_matches, so an album taken
 // off the list stays off. Once done the listing runs again only for albums
 // an older matcher refused (libraryMatcher), refused albums whose tracks
-// have changed since, and library tracks a re-tag filed under a new album
-// (#5579). A person matches the rest by hand, or leaves them off, in the
+// have changed since, and library tracks a re-tag filed under a new album.
+// A person matches the rest by hand, or leaves them off, in the
 // Collection's Unmatched section (MatchAlbumTo, LeaveAlbumOff).
 func (s *Service) listLibrary(ctx context.Context, res *PassResult) error {
 	q := dbq.New(s.pool)