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/sessionvector.go

@@ -13,7 +13,7 @@ type SessionVector struct {
 	Artists        []string       `json:"artists"`
 	Tags           map[string]int `json:"tags"`
 	RecentTrackIDs []string       `json:"recent_track_ids"`
-	// DeviceClass is the current request's device (#1551), set by the radio
+	// DeviceClass is the current request's device, set by the radio
 	// handler from the user's latest play; drives the device dimension of the
 	// context-affinity term. Empty (omitted) for the daily mixes and for the
 	// snapshot stored on play_events, so it never narrows those.
@@ -55,12 +55,6 @@ func uuidString(u pgtype.UUID) string {
 // multi-genre delimiters (`;`, `,`) used by various tag editors. Trims
 // whitespace; drops empty fragments. Strings with no delimiter come back
 // as a single-element slice.
-//
-// This comment used to blame concatenated inputs like
-// "ElectronicComplextroGlitch Hop" on broken tag editors. They were ours: the
-// scanner stored dhowden/tag's welded multi-value frames verbatim. Fixed in
-// #2499 — the scanner now writes ";"-delimited values, so such tokens only
-// survive on rows not yet re-scanned.
 func splitGenres(s string) []string {
 	parts := strings.FieldsFunc(s, func(r rune) bool {
 		return r == ';' || r == ','