Move the archive files into their own package (closes #541)

The SQLite archive files the database target writes are now managed by
internal/archive: file names, rotation, the writer of each target,
pruning, renames, the gzipped JSON export, the idle sweep, and the
lock renames run under, which was Handlers.renameMu. The database
target stays in internal/delivery and writes through archive.Writers;
the engine still closes the writers when it stops, so the stop order
is unchanged. The handlers take the lock, rename and evict through
archive.Archives.

Exported names drop their Archive prefix in the new package. The
tests moved with the code; the two that check a delivery's recorded
attempt stay in delivery.

Model: opus-5-5

internal/handlers/gormlogbound_test.go

@@ -374,7 +374,7 @@ func assertFloodBounded(t *testing.T, label, out string) {
 // this service's three gorm.Open sites: the main database behind the
 // two unauthenticated lookups, and the per-webhook database the
 // WebhookDBManager opens. The third, the archive writer, is pinned in
-// internal/delivery, where its type lives.
+// internal/archive, where its type lives.
 //
 // What each assertion is worth, since two of the three would pass
 // against a service that had never been fixed if the capture were set