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/source_delete_test.go

@@ -16,8 +16,8 @@ import (
 	"github.com/stretchr/testify/require"
 	"gorm.io/gorm"
 	"gorm.io/gorm/clause"
+	"sneak.berlin/go/webhooker/internal/archive"
 	"sneak.berlin/go/webhooker/internal/database"
-	"sneak.berlin/go/webhooker/internal/delivery"
 	"sneak.berlin/go/webhooker/internal/handlers"
 	"sneak.berlin/go/webhooker/internal/session"
 )
@@ -180,7 +180,7 @@ func failSaveOnTable(
 }
 
 // archivePathFor returns the archive database path the
-// delivery engine would use for a database target: beside the
+// archive writers would use for a database target: beside the
 // webhook's event database in the data directory.
 func archivePathFor(
 	t *testing.T,
@@ -192,7 +192,7 @@ func archivePathFor(
 
 	return filepath.Join(
 		filepath.Dir(mgr.DBPath(wh.ID)),
-		delivery.ArchiveFileName(wh.Name, tgt.Name, tgt.ID),
+		archive.FileName(wh.Name, tgt.Name, tgt.ID),
 	)
 }
 
@@ -228,9 +228,9 @@ func postRequest(
 }
 
 // TestHandleSourceDelete_EvictsArchiveWriter proves that
-// deleting a webhook reaches the delivery engine and releases
-// the webhook's archive writers, exercised through the real
-// deletion handler rather than by calling the engine directly.
+// deleting a webhook releases the webhook's archive writers,
+// exercised through the real deletion handler rather than by
+// calling the archive writers directly.
 func TestHandleSourceDelete_EvictsArchiveWriter(t *testing.T) {
 	t.Parallel()
 
@@ -290,7 +290,7 @@ func TestHandleSourceDelete_KeepsArchiveFile(t *testing.T) {
 	wh := seedWebhook(t, db)
 	tgt := seedTarget(t, db, wh.ID, database.TargetTypeDatabase)
 
-	// Place an archive file where the delivery engine would.
+	// Place an archive file where the archive writers would.
 	archivePath := archivePathFor(t, mgr, wh, tgt)
 	require.NoError(
 		t,