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/sqlite_open.go

@@ -118,9 +118,8 @@ const SQLiteFilePerm fs.FileMode = 0o600
 // SQLite still reports the absence rather than this function
 // materializing an empty database the caller asked not to create.
 //
-// Chmod of a file that already exists is what tightens a data
-// directory an earlier build left at 0644 — including a developer's
-// own scratch directory — without any migration machinery.
+// The mode is set on every open, whoever created the file, so a
+// database or sidecar found at a wider mode is tightened too.
 func reserveSQLiteFile(path string, create bool) error {
 	if create {
 		// gosec G304: the path is the database file the caller asked