Show a database target's archive file, size and last write in the target list (closes #397)

For a database target, the target list showed only its expiry, so the archive file the README's backup and move-away advice depend on could only be found from a shell on the host. Each database target now shows its archive file's name, its size on disk (the file and its -wal together) and when it was last written, relative with the full UTC time on hover, all from the files' metadata without opening the archive. Before the first write, and after the file has been moved away, it shows the name and "not created yet". Tests cover all three states.

Model: opus-5-5

internal/delivery/target_database_test.go

@@ -3,6 +3,7 @@ package delivery_test
 import (
 	"database/sql"
 	"fmt"
+	"io/fs"
 	"log/slog"
 	"os"
 	"path/filepath"
@@ -183,6 +184,49 @@ func TestArchiveWriter_RecreatesAfterRemoval(
 	assert.Equal(t, "b", got[0].EventID)
 }
 
+// TestStatArchive proves StatArchive finds no file before the first
+// write; after a write still held in the -wal, counts the -wal in the
+// size and takes its later time as the last write; and finds no file
+// again once the file has been moved away.
+func TestStatArchive(t *testing.T) {
+	t.Parallel()
+
+	path := filepath.Join(t.TempDir(), "archive-wh.db")
+
+	_, err := delivery.StatArchive(path)
+	require.ErrorIs(t, err, fs.ErrNotExist)
+
+	// With the clock stopped, the reopen debounce never passes, so
+	// the handle stays open after the write.
+	stopped := time.Now()
+	w := delivery.NewExportArchiveWriter(path, archiveTestLogger(), 0)
+	w.SetNow(func() time.Time { return stopped })
+
+	require.NoError(t, w.Write(delivery.ExportArchivedEvent{EventID: "a"}, 0))
+
+	written := time.Date(2026, 1, 2, 3, 4, 5, 0, time.UTC)
+	earlier := written.Add(-time.Hour)
+	require.NoError(t, os.Chtimes(path, earlier, earlier))
+	require.NoError(t, os.Chtimes(path+"-wal", written, written))
+
+	file, err := os.Stat(path)
+	require.NoError(t, err)
+
+	wal, err := os.Stat(path + "-wal")
+	require.NoError(t, err)
+	require.Positive(t, wal.Size())
+
+	got, err := delivery.StatArchive(path)
+	require.NoError(t, err)
+	assert.Equal(t, file.Size()+wal.Size(), got.Size)
+	assert.True(t, written.Equal(got.Written), got.Written)
+
+	removeArchiveFiles(t, path)
+
+	_, err = delivery.StatArchive(path)
+	require.ErrorIs(t, err, fs.ErrNotExist)
+}
+
 func TestArchiveWriter_ReopenDebounce(t *testing.T) {
 	t.Parallel()