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/import_markdown_test.go

@@ -204,7 +204,7 @@ func TestImportBackupZip(t *testing.T) {
 	seedAnna(t, store, anna)
 	exp, _ := store.ExportUserData(anna.ID)
 	var buf bytes.Buffer
-	if err := WriteBackupZip(exp, &buf); err != nil {
+	if err := store.WriteBackupZip(anna.ID, exp, &buf); err != nil {
 		t.Fatalf("WriteBackupZip: %v", err)
 	}
 	if buf.Len() == 0 {