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/recommendation/shuffle.go

@@ -16,8 +16,7 @@ type Candidate struct {
 }
 
 // DiversityCaps bounds how much of one selection a single artist or album
-// may occupy. A zero value means "no cap", which is what Shuffle did
-// unconditionally before #3882.
+// may occupy. A zero value means "no cap".
 type DiversityCaps struct {
 	MaxPerArtist int
 	MaxPerAlbum  int
@@ -53,7 +52,7 @@ func RadioDiversityCaps(limit int) DiversityCaps {
 }
 
 // heldBack says whether the candidate is a video rip held back from radio
-// (M498 #5439): it plays when chosen, but radio does not choose it.
+// it plays when chosen, but radio does not choose it.
 func heldBack(c Candidate) bool {
 	return c.Track.SourceVerdict != nil && *c.Track.SourceVerdict == "suspect"
 }
@@ -69,19 +68,13 @@ func heldBack(c Candidate) bool {
 // So the result holds min(limit, len(candidates)) either way — the caps
 // change WHICH tracks are chosen, never HOW MANY. (What is not a candidate at
 // all is dropped first: a held-back video rip, and a second copy of a song
-// already chosen from another release, M498.)
+// already chosen from another release.)
 //
-// That two-pass shape is the whole design, and a hard cap would have been
-// the easy mistake. Radio asks for 50 tracks by default and 200 at most; a
-// pool concentrated on a few artists would return six tracks and call it a
-// radio. Rule 131's principle — degrade, never vanish — applies past the
-// system mixes it was written for.
-//
-// Before #3882 there was no cap here at all, while discover.go,
-// you_might_like.go and home.go all had one. That asymmetry is what let a
-// radio session come back entirely from a single artist: nothing between
-// the pool and the output bounded any artist's share, so a pool dominated
-// by one artist produced an output dominated by it too.
+// That two-pass shape is the whole design. A hard cap would be the easy
+// mistake: radio asks for 50 tracks by default and 200 at most, and a pool
+// concentrated on a few artists would return six tracks and call it a radio.
+// Degrade, never vanish. Without any cap, a pool dominated by one artist
+// would produce a radio entirely of that artist.
 func Shuffle(
 	candidates []Candidate,
 	weights ScoringWeights,
@@ -112,7 +105,7 @@ func Shuffle(
 	deferred := make([]Candidate, 0, len(scored)-limit)
 	artistCount := map[pgtype.UUID]int{}
 	albumCount := map[pgtype.UUID]int{}
-	// One song on several releases is played once (M498 #5438): after its
+	// One song on several releases is played once: after its
 	// best-scoring copy, the others are dropped, not held back.
 	songs := map[pgtype.UUID]bool{}
 	repeat := func(c Candidate) bool { return c.Track.SongKey.Valid && songs[c.Track.SongKey] }