Describe code in its own terms, not by an earlier build's data (closes #522)

Several comments justified code by what an earlier build would have
left in a database or on disk. Each now says what the code does:
reserveSQLiteFile sets the mode on every open whoever created the
file, and expired treats a session lacking a deadline's timestamp as
expired. The comments on retention, resubmitted events, archive
pruning, target headers, the redactor and the delivery result view
drop their mentions of older rows. Comments only.

Model: opus-5-5

internal/database/model_webhook.go

@@ -98,8 +98,8 @@ func (w *Webhook) BeforeSave(_ *gorm.DB) error {
 // are exempt from reaping.
 //
 // It accepts the RetentionForeverDays sentinel written by BeforeSave
-// and, defensively, the non-positive values that rows written before
-// the sentinel existed may still carry.
+// and, defensively, any non-positive value, which BeforeSave treats
+// as a request to retain forever.
 func retainsForever(retentionDays int) bool {
 	return retentionDays <= 0 ||
 		retentionDays >= RetentionForeverDays