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/delivery/archive_sweeper.go

@@ -201,8 +201,7 @@ func (s *ArchiveSweeper) sweep(ctx context.Context) {
 
 // sweepTarget prunes the archive of a single database target.
 // A missing, empty, or "never" expiry parses as a zero duration
-// and is skipped entirely, so those archives keep exactly the
-// behaviour they had before the sweep existed.
+// and is skipped entirely, so those archives are never pruned.
 func (s *ArchiveSweeper) sweepTarget(target *database.Target) {
 	expiry, err := parseArchiveExpiry(target.Config)
 	if err != nil {