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.go

@@ -2,6 +2,7 @@ package storage
 
 import (
 	"archive/zip"
+	"bytes"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -10,6 +11,7 @@ import (
 	"regexp"
 	"sort"
 	"strings"
+	"time"
 	"unicode/utf8"
 
 	"github.com/sandrodach/mindsift/internal/domain"
@@ -300,6 +302,63 @@ func PlanMindSiftArchiveImport(r io.ReaderAt, size int64) (*ImportPlan, error) {
 	return plan, nil
 }
 
+// BackupFileName is the JSON inside the backup ZIP.
+const BackupFileName = "mindsift-backup.json"
+
+// WriteBackupZip streams the JSON backup compressed into a ZIP with a single file.
+func WriteBackupZip(exp *domain.FullExport, w io.Writer) error {
+	zw := zip.NewWriter(w)
+	f, err := zw.CreateHeader(&zip.FileHeader{Name: BackupFileName, Method: zip.Deflate, Modified: parseISO(exp.ExportedAt, time.Now())})
+	if err != nil {
+		return err
+	}
+	enc := json.NewEncoder(f)
+	enc.SetIndent("", "  ")
+	if err := enc.Encode(exp); err != nil {
+		return err
+	}
+	return zw.Close()
+}
+
+// PlanMindSiftImport picks the format of MindSift's own data: plain JSON backup, the backup ZIP
+// (one JSON inside) or the Markdown archive.
+func PlanMindSiftImport(data []byte, fileName string) (*ImportPlan, error) {
+	if strings.HasSuffix(strings.ToLower(fileName), ".json") || (len(data) > 0 && data[0] == '{') {
+		return PlanMindSiftJSONImport(data)
+	}
+	zr, err := zip.NewReader(bytes.NewReader(data), int64(len(data)))
+	if err != nil {
+		return nil, errors.New("Die Datei ist weder ein MindSift-Backup (.json/.zip) noch ein MindSift-Markdown-Archiv")
+	}
+	var backup *zip.File
+	for _, f := range zr.File {
+		name := cleanArchivePath(f.Name)
+		if strings.EqualFold(path.Ext(name), ".md") {
+			backup = nil
+			break
+		}
+		if strings.EqualFold(path.Ext(name), ".json") && !hiddenPath(name) {
+			backup = f
+		}
+	}
+	if backup == nil {
+		return PlanMindSiftArchiveImport(bytes.NewReader(data), int64(len(data)))
+	}
+	if backup.UncompressedSize64 > maxImportUnpackedBytes {
+		return nil, errors.New("Backup ist entpackt zu groß (max. 500 MB)")
+	}
+	rc, err := backup.Open()
+	if err != nil {
+		return nil, errors.New("Backup im Archiv ist nicht lesbar")
+	}
+	defer rc.Close()
+	raw, err := io.ReadAll(io.LimitReader(rc, maxImportUnpackedBytes+1))
+	if err != nil || len(raw) > maxImportUnpackedBytes {
+		return nil, errors.New("Backup im Archiv ist nicht lesbar oder zu groß")
+	}
+	return PlanMindSiftJSONImport(raw)
+}
+
 // PlanMindSiftJSONImport reads the JSON backup ("Backup herunterladen"). Notes shared with the
 // exporting user belong to someone else and are skipped; discarded thoughts are not restored.
 func PlanMindSiftJSONImport(data []byte) (*ImportPlan, error) {