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/storage/migrations.go

@@ -26,6 +26,7 @@ var migrations = []migration{
 	{3, "Basis-Schema v0.4.1", (*Store).migrateBaseline},
 	{4, "Sitzungen", (*Store).migrateSessions},
 	{5, "Medien", (*Store).migrateMedia},
+	{6, "Link-Vorschauen", (*Store).migrateLinkPreviews},
 }
 
 func (s *Store) migrate() error {