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/score.go

@@ -1,5 +1,5 @@
-// Package recommendation implements the weighted-shuffle scoring engine
-// from spec §6. The Score function is pure and takes an injectable RNG so
+// Package recommendation implements the weighted-shuffle scoring engine.
+// The Score function is pure and takes an injectable RNG so
 // tests can pin jitter to deterministic values.
 package recommendation
 
@@ -20,20 +20,20 @@ type ScoringInputs struct {
 	ContextualMatchScore float64    // [0, 1]; 0 when no signal
 	SimilarityScore      float64    // [0, 1]; 0 when no signal (random fill)
 	// TasteMatchScore is the candidate's fit to the user's learned taste
-	// profile (#796 phase 2), in [-1, +1]: positive draws a track toward the
+	// profile, in [-1, +1]: positive draws a track toward the
 	// user's taste, negative reflects passive avoidance, 0 when there's no
 	// profile signal (cold start / artist+tags absent from the profile).
 	TasteMatchScore float64
 	// ContextAffinityScore is the candidate artist's time-of-day/weekday
-	// affinity for the CURRENT context (#1531), in [-1, +1]: positive when the
+	// affinity for the CURRENT context, in [-1, +1]: positive when the
 	// artist's plays concentrate in the current daypart × weekday-type cell
 	// more than the user's baseline, negative when under-represented, 0 when
 	// there's no history (cold-start neutral).
 	ContextAffinityScore float64
 }
 
-// ScoringWeights are the operator-tunable knobs. Defaults live in
-// config.RecommendationConfig and are propagated here per request.
+// ScoringWeights are the tunable knobs. The shipped values and any tuned ones
+// live in internal/recsettings, which hands them in per request.
 type ScoringWeights struct {
 	BaseWeight        float64
 	LikeBoost         float64
@@ -46,7 +46,7 @@ type ScoringWeights struct {
 	ContextTimeWeight float64
 }
 
-// Score computes the weighted-shuffle score per spec §6:
+// Score computes the weighted-shuffle score:
 //
 //	score = base
 //	      + (is_general_liked ? LikeBoost : 0)