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/playlists/service.go

@@ -1,6 +1,6 @@
 // Package playlists implements the user-facing playlist CRUD service.
 //
-// Visibility model (from the M7 #352 design spec):
+// Visibility model:
 //   - Playlists are private by default. Only the owner sees private rows
 //     in List, Get, and is allowed to mutate them via Update / Delete.
 //   - is_public = true makes a playlist readable by any authenticated
@@ -112,7 +112,7 @@ type PlaylistTrack struct {
 	AddedAt     pgtype.Timestamptz
 	// Unavailable reports that the track still exists in the library but
 	// its file is currently missing from disk (tracks.missing_since is
-	// set, #2527). The entry keeps its place in the playlist — the user
+	// set). The entry keeps its place in the playlist — the user
 	// put it there — but nothing should try to play it.
 	Unavailable bool
 }