feat(workspace): link previews above bullets with a web address (M5)

- GET /link-preview?url= fetches the page on the server (https only,
  SSRF-guarded like webhooks), reads Open Graph/Twitter tags, title,
  description and icon from the head, loads picture and icon (no SVG) and
  keeps them in link_previews (migration 6) for 7 days; readers' browsers
  never contact the page. Off in the demo; 300 fetches per address in
  15 minutes.
- Compact card above the bullet (icon, site, title, one line, small
  picture); the duplicate link badge is left out.
- ✕ turns the address into [title](url), so hiding syncs with the note;
  "Link-Vorschauen anzeigen" under appearance switches all off per device.

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

internal/api/docsync.go

@@ -271,6 +271,7 @@ func (s *Server) startCompaction() {
 func (s *Server) compactOnce() {
 	s.leases.Sweep()
 	s.compactMedia(time.Now())
+	_ = s.store.PruneLinkPreviews(time.Now().Add(-30 * 24 * time.Hour))
 	st, err := s.store.CompactDocs(time.Now())
 	if err != nil {
 		slog.Error("Verdichtung des Änderungsprotokolls fehlgeschlagen", "error", err)