feat(media): images and recordings in backups, exports and imports (M6)

- Backup ZIP (download and automatic per-user server backups) carries the
  files of the user's own notes as media/<id><ext>; restoring creates
  them with new IDs and rewrites the blocks.
- Markdown archive puts them under attachments/ and embeds them relative
  to each note (![caption](../attachments/…)); re-importing turns them
  back into image/audio blocks.
- Obsidian import takes images and audio a note embeds on its own
  (![[file.png]], ![](path/file.png)); unused or invalid files appear as
  skipped in the preview instead of vanishing.
- Format detection and file paths moved into storage for this.

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

internal/storage/export_markdown.go

@@ -32,15 +32,36 @@ func (s *Store) WriteMarkdownVault(userID string, w io.Writer) error {
 		}
 	}
 
+	// Images and recordings go to attachments/, notes link them relative to their folder
+	attachments := map[string]string{} // media ID -> path in the archive
+	if media, err := s.MediaOfUser(userID); err == nil {
+		for _, m := range media {
+			attachments[m.ID] = vaultAttachmentDir + "/" + m.ID + MediaExt(m.Mime)
+		}
+	}
+	if err := s.writeMediaFiles(zw, userID, func(m Media) string { return attachments[m.ID] }); err != nil {
+		return err
+	}
+
 	docs, err := s.GetDocuments(userID)
 	if err != nil {
 		return err
 	}
 	for _, d := range docs {
-		name := names.file(names.dir(d.FolderPath), d.Title, "Unbenannte Notiz")
+		dir := names.dir(d.FolderPath)
+		name := names.file(dir, d.Title, "Unbenannte Notiz")
+		up := ""
+		if dir != "" {
+			up = strings.Repeat("../", strings.Count(dir, "/")+1)
+		}
 		body := markdownFrontmatter(map[string]any{
 			"title": d.Title, "icon": d.Icon, "tags": d.Tags, "created_at": d.CreatedAt, "updated_at": d.UpdatedAt,
-		}) + BlocksToVaultMarkdown(d.Blocks)
+		}) + blocksToVaultMarkdown(d.Blocks, func(id string) string {
+			if p, ok := attachments[id]; ok {
+				return up + p
+			}
+			return ""
+		})
 		if err := writeZipFile(zw, name, body, parseISO(d.UpdatedAt, now)); err != nil {
 			return err
 		}
@@ -164,7 +185,16 @@ func taskMarkdownLine(t domain.Thought) string {
 // BlocksToVaultMarkdown turns outliner blocks into Markdown that Obsidian renders natively:
 // top-level headings, dividers and callouts as real block elements, everything else as a nested
 // list (one tab per level), code as fenced blocks inside the list.
+// vaultAttachmentDir holds images and recordings in the Markdown archive.
+const vaultAttachmentDir = "attachments"
+
 func BlocksToVaultMarkdown(blocks []domain.DocumentBlock) string {
+	return blocksToVaultMarkdown(blocks, nil)
+}
+
+// blocksToVaultMarkdown renders blocks as Markdown; attachment maps a media ID to its relative path
+// (image and audio blocks become embeds Obsidian shows inline).
+func blocksToVaultMarkdown(blocks []domain.DocumentBlock, attachment func(id string) string) string {
 	var sb strings.Builder
 	// Children of a top-level heading/callout/code/divider become a list one level up; otherwise their
 	// tab indent would turn them into an indented code block in Markdown
@@ -178,6 +208,13 @@ func BlocksToVaultMarkdown(blocks []domain.DocumentBlock) string {
 		}
 		indent := strings.Repeat("\t", lvl)
 		content := strings.ReplaceAll(b.Content, "\r\n", "\n")
+		if (b.Type == "image" || b.Type == "audio") && attachment != nil {
+			if ref, ok := ParseMediaContent(content); ok {
+				if p := attachment(ref.ID); p != "" {
+					content = "![" + ref.Caption + "](" + strings.ReplaceAll(p, " ", "%20") + ")"
+				}
+			}
+		}
 		// Soft line breaks continue the list item, aligned under its text
 		cont := func(text, prefix string) string {
 			return strings.ReplaceAll(text, "\n", "\n"+indent+prefix)