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/context.go

@@ -15,7 +15,7 @@ import (
 const contextAffinityShrinkage = 5.0
 
 // ContextAffinity is the read-side map of per-artist time-of-day/weekday
-// affinity for the CURRENT context (#1531): artist_id → score in [-1, +1].
+// affinity for the CURRENT context: artist_id → score in [-1, +1].
 // Absent artists (no play history) score 0, so cold-start candidates stay
 // neutral. The zero value is a valid all-neutral affinity.
 type ContextAffinity struct {
@@ -29,7 +29,7 @@ func (c ContextAffinity) Affinity(artistID pgtype.UUID) float64 {
 
 // LoadContextAffinity computes each artist's affinity for the user's CURRENT
 // context cell — daypart × weekday, narrowed by deviceClass when non-empty
-// (#1551; radio passes the current device, the daily mixes pass "" for a
+// (radio passes the current device, the daily mixes pass "" for a
 // device-agnostic cell). For every artist with completed plays in the window it
 // compares the share of that artist's plays that fall in the current cell
 // against the user's overall baseline share, shrinking sparse artists toward