docs: api comments say what the endpoints do, not how they came to be (#6277)

Rewrites the internal/api comment blocks to rule 32: task, milestone and
rule ids gone, the stories behind them restated as the constraint they
left, stale references corrected (the stream secret is wired from config,
the timezone handler already refreshes the scheduler, the Android app is
the native client, the design docs cited are not in the repo). Also
rewraps comment lines over 80 columns left by the earlier batches.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/playlists_system_shuffle.go

@@ -14,15 +14,14 @@ import (
 	"git.fabledsword.com/bvandeusen/minstrel/internal/playlists"
 )
 
-// Generic registry-driven system-playlist endpoints (#411 R2),
-// replacing the former per-kind handlers. {kind} is the raw
+// Generic registry-driven system-playlist endpoints. {kind} is the raw
 // system_variant ('for_you' | 'discover' | future singleton kinds);
 // only singleton kinds (playlists.RefreshableSystemKind) are
 // addressable here — songs_like_artist is multi-per-user and played
 // by playlist id, so it 404s.
 //
 // GET /api/playlists/system/{kind}/shuffle — the caller's playlist
-// for that kind with tracks in rotation-aware order (#415): unheard
+// for that kind with tracks in rotation-aware order: unheard
 // this rotation first (shuffled), then heard; resets when exhausted.
 // Same JSON shape as GET /api/playlists/{id} so clients reuse track
 // parsing. Intentionally uncached — it varies per play, unlike the
@@ -73,7 +72,7 @@ func (h *handlers) handleSystemPlaylistRefresh(w http.ResponseWriter, r *http.Re
 		writeErr(w, apierror.InternalMsg("build failed", err))
 		return
 	}
-	// #968: announce the rebuild so the user's other clients refresh.
+	// Announce the rebuild so the user's other clients refresh.
 	h.publishSystemRebuilt(user.ID)
 	q := dbq.New(h.pool)
 	v := kind