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/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 == "" {