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/delivery/engine_lifecycle_test.go

@@ -427,9 +427,9 @@ func TestEngine_StopLeavesLastRetryAttemptRetrying(t *testing.T) {
 
 // deliverToArchive gives the setup's webhook a database target,
 // runs one delivery to it through the running engine, and returns
-// the target's ID and archive file path. The archive writer holds
-// the file open afterwards.
-func deliverToArchive(t *testing.T, s iSetup) (string, string) {
+// the target's archive file path. The archive writer holds the file
+// open afterwards.
+func deliverToArchive(t *testing.T, s iSetup) string {
 	t.Helper()
 
 	iCreateWebhook(t, s.MainDB, s.WebhookID, "hook")
@@ -452,7 +452,7 @@ func deliverToArchive(t *testing.T, s iSetup) (string, string) {
 
 	iWaitForDelivered(t, s.WebhookDB, deliveryID)
 
-	return tgt.ID, filepath.Join(
+	return filepath.Join(
 		filepath.Dir(s.DBMgr.DBPath(s.WebhookID)),
 		"archive-hook-archive-"+tgt.ID+".db",
 	)
@@ -471,7 +471,7 @@ func TestEngine_StopHookClosesArchives(t *testing.T) {
 
 	lc := startEngineViaHook(t, s.Engine)
 
-	_, path := deliverToArchive(t, s)
+	path := deliverToArchive(t, s)
 	require.FileExists(
 		t, path+"-wal",
 		"an open archive should have a -wal for the stop to remove",
@@ -505,21 +505,23 @@ func TestEngine_StopHookTimeoutLeavesArchivesOpen(t *testing.T) {
 
 	lc := startEngineViaHook(t, s.Engine)
 
-	targetID, _ := deliverToArchive(t, s)
+	path := deliverToArchive(t, s)
 
 	release := make(chan struct{})
 
 	t.Cleanup(func() {
 		close(release)
-		s.Engine.EvictWebhook(s.WebhookID)
+		s.Engine.ExportArchives().EvictWebhook(s.WebhookID)
 	})
 
 	s.Engine.ExportWedgeWorker(release)
 
 	requireStopHookExpires(t, lc.hooks[0], "delivery engine")
 
-	require.True(
-		t, s.Engine.ExportArchiveHandleOpen(targetID),
+	// Closing the last handle on an archive removes its -wal, so a
+	// -wal still there is a writer still open.
+	require.FileExists(
+		t, path+"-wal",
 		"a stop that timed out must not close archive writers",
 	)
 }