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/linkpreview.go

@@ -0,0 +1,87 @@
+package storage
+
+import (
+	"crypto/sha256"
+	"database/sql"
+	"encoding/hex"
+	"errors"
+	"time"
+)
+
+// Link previews: title, description, a small picture and the site icon of a web page, fetched once by
+// the server and kept for a while, so readers' browsers never contact the page themselves.
+
+type LinkPreview struct {
+	Key         string `json:"key"`
+	URL         string `json:"url"`
+	Title       string `json:"title"`
+	Description string `json:"description"`
+	SiteName    string `json:"siteName"`
+	HasImage    bool   `json:"hasImage"`
+	HasIcon     bool   `json:"hasIcon"`
+	OK          bool   `json:"ok"` // false: the page gave nothing usable (kept shorter, then tried again)
+	FetchedAt   string `json:"fetchedAt"`
+
+	Image, Icon         []byte `json:"-"`
+	ImageMime, IconMime string `json:"-"`
+}
+
+// LinkPreviewKey identifies the preview of a URL (also used in the picture URLs).
+func LinkPreviewKey(url string) string {
+	sum := sha256.Sum256([]byte(url))
+	return hex.EncodeToString(sum[:12])
+}
+
+func (s *Store) migrateLinkPreviews() error {
+	return s.execInTx(stmt(`CREATE TABLE IF NOT EXISTS link_previews (
+		key TEXT PRIMARY KEY,
+		url TEXT NOT NULL,
+		title TEXT NOT NULL DEFAULT '',
+		description TEXT NOT NULL DEFAULT '',
+		site_name TEXT NOT NULL DEFAULT '',
+		image BLOB,
+		image_mime TEXT NOT NULL DEFAULT '',
+		icon BLOB,
+		icon_mime TEXT NOT NULL DEFAULT '',
+		ok INTEGER NOT NULL DEFAULT 0,
+		fetched_at TEXT NOT NULL
+	)`))
+}
+
+var ErrLinkPreviewNotFound = errors.New("Keine Vorschau vorhanden")
+
+func (s *Store) GetLinkPreview(key string) (*LinkPreview, error) {
+	var p LinkPreview
+	var ok int
+	err := s.db.QueryRow(`SELECT key, url, title, description, site_name, image, image_mime, icon, icon_mime, ok, fetched_at
+		FROM link_previews WHERE key = ?`, key).
+		Scan(&p.Key, &p.URL, &p.Title, &p.Description, &p.SiteName, &p.Image, &p.ImageMime, &p.Icon, &p.IconMime, &ok, &p.FetchedAt)
+	if errors.Is(err, sql.ErrNoRows) {
+		return nil, ErrLinkPreviewNotFound
+	}
+	if err != nil {
+		return nil, err
+	}
+	p.OK = ok == 1
+	p.HasImage, p.HasIcon = len(p.Image) > 0, len(p.Icon) > 0
+	return &p, nil
+}
+
+func (s *Store) SaveLinkPreview(p *LinkPreview) error {
+	p.Key = LinkPreviewKey(p.URL)
+	p.FetchedAt = time.Now().UTC().Format(time.RFC3339)
+	_, err := s.db.Exec(`INSERT INTO link_previews (key, url, title, description, site_name, image, image_mime, icon, icon_mime, ok, fetched_at)
+		VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)
+		ON CONFLICT(key) DO UPDATE SET url = excluded.url, title = excluded.title, description = excluded.description,
+			site_name = excluded.site_name, image = excluded.image, image_mime = excluded.image_mime, icon = excluded.icon,
+			icon_mime = excluded.icon_mime, ok = excluded.ok, fetched_at = excluded.fetched_at`,
+		p.Key, p.URL, p.Title, p.Description, p.SiteName, p.Image, p.ImageMime, p.Icon, p.IconMime, boolInt(p.OK), p.FetchedAt)
+	p.HasImage, p.HasIcon = len(p.Image) > 0, len(p.Icon) > 0
+	return err
+}
+
+// PruneLinkPreviews forgets previews older than the given time (they are fetched again when needed).
+func (s *Store) PruneLinkPreviews(olderThan time.Time) error {
+	_, err := s.db.Exec(`DELETE FROM link_previews WHERE fetched_at < ?`, olderThan.UTC().Format(time.RFC3339))
+	return err
+}