feat(backup): download the full backup as ZIP

GET /export?format=zip streams the JSON backup compressed (plain JSON stays
the API default). The MindSift import recognises the backup ZIP next to the
plain JSON and the Markdown archive.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/storage/import_markdown_test.go

@@ -197,3 +197,25 @@ func TestImportObsidianVault(t *testing.T) {
 		t.Errorf("invalid archive must be rejected")
 	}
 }
+
+func TestImportBackupZip(t *testing.T) {
+	store, anna, ben := newImportStore(t)
+	seedAnna(t, store, anna)
+	exp, _ := store.ExportUserData(anna.ID)
+	var buf bytes.Buffer
+	if err := WriteBackupZip(exp, &buf); err != nil {
+		t.Fatalf("WriteBackupZip: %v", err)
+	}
+	if buf.Len() == 0 {
+		t.Fatalf("empty archive")
+	}
+	plan, err := PlanMindSiftImport(buf.Bytes(), "mindsift-backup-2026-10-29.zip")
+	if err != nil {
+		t.Fatalf("plan: %v", err)
+	}
+	if _, err := store.ApplyImport(ben.ID, plan); err != nil {
+		t.Fatalf("apply: %v", err)
+	}
+	checkRestored(t, store, ben)
+}
+