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. parseMaxRetries loses its
paragraph on targets saved before the ceiling existed, and neither
BeforeSave nor retentionCutoff cites a migration. Comments only.

Model: opus-5-5

internal/handlers/delivery_result_view.go

@@ -12,13 +12,12 @@ import (
 //
 // The bound is enforced here and in SQL, because this page's
 // memory profile must not depend on a constant in another
-// package staying where it is, and because rows predating the
-// delivery engine's own cap or restored from an archive are
-// not covered by it at all.
+// package staying where it is, or on every stored row having
+// been written by the delivery engine under its own cap.
 //
 // It happens to equal that engine cap, so a row written by the
-// current engine reaches this one exactly and is never cut
-// twice. Nothing here may assume the two differ: see view.
+// engine reaches this one exactly and is never cut twice.
+// Nothing here may assume the two differ: see view.
 const maxRenderedResponseBytes = 4096
 
 // deliveryResultColumns is the delivery attempt projection.
@@ -40,9 +39,8 @@ const deliveryResultColumns = "delivery_id, attempt_num, created_at, " +
 //
 // Both Error and ResponseBody have been through the target's
 // Redactor. The engine already masks the URL out of the
-// errors it stores, so for errors this is a second line
-// covering rows written before it did; for response bodies it
-// is the only line, and its reach is what
+// errors it stores, so for errors this is a second line; for
+// response bodies it is the only line, and its reach is what
 // delivery.Redactor documents.
 type DeliveryResultView struct {
 	AttemptNum int

internal/handlers/target_retries.go

@@ -49,13 +49,6 @@ var (
 // working retry configuration over a typo. A default answers a
 // question that was not asked; it never answers one that was asked
 // badly.
-//
-// A target stored with a count above the ceiling before this
-// validation existed keeps rendering and keeps delivering — nothing
-// clamps the row. Re-saving it from the edit form does have to bring
-// it into range, because the form submits the pre-filled value back
-// and accepting it would be the ceiling not applying to the edit
-// path. The 400 names the ceiling, so the fix is one field.
 func parseMaxRetries(raw string, fallback int) (int, error) {
 	raw = strings.TrimSpace(raw)
 	if raw == "" {