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/taste.go

@@ -16,11 +16,11 @@ import (
 // rest toward zero (as a per-user max-normalisation would). A weight at the
 // scale value maps to tanh(1) ≈ 0.76 — "clearly a preference."
 //
-// The era (#1530) and mood (#1534) terms are added on TOP of artist+tag
+// The era and mood terms are added on TOP of artist+tag
 // (shares don't sum to 1) so they're pure nudges that never weaken the existing
 // artist/tag signal when a track lacks a date or mood tags; clampUnit bounds
 // the combined result. Their shares are small — both are coarse, partial-
-// coverage signals — and re-bakeable if the lab shows they should carry more.
+// coverage signals — and re-bakeable if tuning shows they should carry more.
 const (
 	tasteArtistScale  = 4.0
 	tasteTagScale     = 3.0