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/inflight_test.go

@@ -2,8 +2,6 @@ package delivery_test
 
 import (
 	"context"
-	"path/filepath"
-	"strings"
 	"sync"
 	"testing"
 	"time"
@@ -66,33 +64,6 @@ func fDrain(e *delivery.Engine) []delivery.Task {
 	}
 }
 
-// TestArchiveHandleIsWAL closes the last gap in the durability
-// evidence: the main and per-webhook tiers each assert their journal
-// mode on a live handle, and the archive tier gets its settings from
-// the same code path but nothing checked the running file.
-func TestArchiveHandleIsWAL(t *testing.T) {
-	t.Parallel()
-
-	w := delivery.NewExportArchiveWriter(
-		filepath.Join(t.TempDir(), "archive-wal.db"),
-		archiveTestLogger(), 0,
-	)
-
-	require.NoError(t, w.Open(0))
-
-	var mode string
-
-	row := w.DB().Raw("pragma journal_mode").Row()
-	require.NoError(t, row.Scan(&mode))
-	assert.Equal(t, "wal", strings.ToLower(mode))
-
-	var busy string
-
-	row = w.DB().Raw("pragma busy_timeout").Row()
-	require.NoError(t, row.Scan(&busy))
-	assert.Equal(t, "10000", busy)
-}
-
 // TestSweepLeavesAQueuedDeliveryAlone is the case the age bound cannot
 // see. The delivery is queued and untouched, so its row is arbitrarily
 // old and still perfectly healthy; only ownership distinguishes it