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/suggestions.go

@@ -4,11 +4,11 @@
 // through artist_similarity_unmatched via a single CTE, and returns top-N
 // out-of-library candidates with top-3 attribution seeds resolved to names.
 //
-// Originally M5c, which seeded from raw likes + plays. That signal grew
-// without bound and counted skips as engagement, so a few heavily-played
-// artists monopolized every slot and the surface entrenched harder the more
-// the user listened. Reworked to seed from the taste profile in issue #2367;
-// see internal/db/queries/recommendation.sql for the tiering.
+// Seeding from the taste profile rather than raw likes + plays matters: raw
+// counts grow without bound and count skips as engagement, so a few
+// heavily-played artists would monopolize every slot and entrench harder the
+// more the user listened. See internal/db/queries/recommendation.sql for the
+// tiering.
 package recommendation
 
 import (
@@ -33,12 +33,12 @@ type ArtistSuggestion struct {
 	Attribution []SeedContribution
 	// MatchedTags are the candidate's own tags that overlap the user's taste
 	// profile, strongest first (max 3) — the "matches: shoegaze, melancholic"
-	// explanation (#2377). Empty when the candidate has no cached tags, which
-	// is common and not an error: coverage is permanently partial (#2376).
+	// explanation. Empty when the candidate has no cached tags, which is
+	// common and not an error: coverage is permanently partial.
 	MatchedTags []string
 	// TagOverlap is the [0,1] share of the candidate's tag mass the user likes.
-	// Exposed for the admin tuning lab — seeing the term's actual distribution
-	// is how the operator picks a weight rather than guessing at one.
+	// Exposed for admin tuning — seeing the term's actual distribution is how
+	// a weight gets picked rather than guessed.
 	TagOverlap float64
 }
 
@@ -71,8 +71,8 @@ func SuggestArtists(
 	q := dbq.New(pool)
 	// Over-fetch so there is something to rotate through. A deterministic
 	// top-N over a score ordering shows the same faces every day until one is
-	// requested away — the tail of the ranking was unreachable (#2367
-	// mechanism 1). Selection from this pool happens in selectSuggestions.
+	// requested away, leaving the tail of the ranking unreachable. Selection
+	// from this pool happens in selectSuggestions.
 	rows, err := q.SuggestArtistsForUser(ctx, dbq.SuggestArtistsForUserParams{
 		UserID:  userID,
 		Column2: halfLifeDays,
@@ -130,7 +130,7 @@ func SuggestArtists(
 		})
 	}
 
-	// Taste-tag term (#2377). Applied to the whole over-fetched pool BEFORE
+	// Taste-tag term. Applied to the whole over-fetched pool BEFORE
 	// selection, so the rotation and diversity rules in selectSuggestions
 	// operate on taste-blended scores — boosting only the twelve already
 	// chosen by similarity would leave the actual re-ranking undone.
@@ -227,9 +227,8 @@ func rotationDay(now time.Time) string {
 // Three rules, in order:
 //
 //  1. Diversity cap. Walking in score order, a candidate is dropped once its
-//     dominant seed already owns maxPerSeed slots. Nothing capped this before,
-//     so all twelve slots could be neighbours of one artist and the surface
-//     read as a single narrow cluster (#2367 mechanism 4).
+//     dominant seed already owns maxPerSeed slots, so twelve slots cannot all
+//     be neighbours of one artist and read as a single narrow cluster.
 //  2. Head. The best few by score always lead, so the strongest matches are
 //     never rotated out of sight. Mirrors For You's head/tail shape.
 //  3. Tail. The rest of the slots are drawn from the remaining pool ordered by
@@ -263,8 +262,8 @@ func selectSuggestions(pool []ArtistSuggestion, limit int, day string) []ArtistS
 	}
 	// Diversity is a PREFERENCE, not a quota that may starve the deck. A user
 	// whose whole pool hangs off one or two seeds would otherwise get a
-	// three-card surface, which is worse than the monoculture we were avoiding
-	// (and is the vanish-or-nothing shape rule #131 exists to prevent). Top up
+	// three-card surface, which is worse than the monoculture it avoids.
+	// Top up
 	// in score order from what the cap set aside.
 	for _, s := range overflow {
 		if len(out) >= limit {