feat(workspace): backlinks at the end of a note

GET /documents/{id}/backlinks lists the notes the user can open whose
blocks link [[Title]] (also |alias and #section, case-insensitive),
excluding the note itself and the trash. The collapsible "Verlinkt auf
diese Notiz" section shows each note with folder and the linking bullets;
a click jumps straight to the bullet.

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

internal/storage/backlinks.go

@@ -0,0 +1,101 @@
+package storage
+
+import (
+	"errors"
+	"regexp"
+	"sort"
+	"strings"
+)
+
+// Backlinks ("Verlinkt auf diese Notiz") lists the notes whose blocks contain a wiki link [[Title]]
+// to a note. Matching ignores case and surrounding spaces like the link click in the editor;
+// [[Title|Text]] and [[Title#Abschnitt]] count as well. Only notes the user can open are listed,
+// never the note itself or notes in the trash.
+
+// ErrNoAccess is returned when the user cannot open the note.
+var ErrNoAccess = errors.New("Kein Zugriff auf diese Notiz")
+
+var wikiLinkRe = regexp.MustCompile(`\[\[([^\]|#]+)(?:[|#][^\]]*)?\]\]`)
+
+const maxBacklinkBlocks = 5
+
+type BacklinkBlock struct {
+	ID      string `json:"id"`
+	Content string `json:"content"`
+}
+
+type Backlink struct {
+	DocID      string          `json:"docId"`
+	Title      string          `json:"title"`
+	Icon       string          `json:"icon"`
+	FolderPath string          `json:"folderPath"`
+	Blocks     []BacklinkBlock `json:"blocks"`
+	More       int             `json:"more,omitempty"` // further matching blocks not listed
+}
+
+func linksTo(content, title string) bool {
+	for _, m := range wikiLinkRe.FindAllStringSubmatch(content, -1) {
+		if strings.EqualFold(strings.TrimSpace(m[1]), title) {
+			return true
+		}
+	}
+	return false
+}
+
+func (s *Store) GetBacklinks(userID, docID string) ([]Backlink, error) {
+	if s.DocRole(userID, docID) == "" {
+		return nil, ErrNoAccess
+	}
+	var title string
+	if err := s.db.QueryRow(`SELECT title FROM documents WHERE id = ?`, docID).Scan(&title); err != nil {
+		return nil, err
+	}
+	title = strings.TrimSpace(title)
+	out := []Backlink{}
+	if title == "" {
+		return out, nil
+	}
+
+	// The tree of the user: what they can open, with their own folder paths
+	docs, err := s.GetDocuments(userID)
+	if err != nil {
+		return nil, err
+	}
+	byID := make(map[string]int, len(docs))
+	for i, d := range docs {
+		byID[d.ID] = i
+	}
+
+	rows, err := s.db.Query(`SELECT id, document_id, content FROM blocks
+		WHERE deleted = 0 AND document_id != ? AND content LIKE '%[[%'
+		ORDER BY document_id, order_key`, docID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	index := map[string]int{}
+	for rows.Next() {
+		var blockID, sourceID, content string
+		if rows.Scan(&blockID, &sourceID, &content) != nil {
+			continue
+		}
+		di, visible := byID[sourceID]
+		if !visible || !linksTo(content, title) {
+			continue
+		}
+		pos, seen := index[sourceID]
+		if !seen {
+			d := docs[di]
+			out = append(out, Backlink{DocID: d.ID, Title: d.Title, Icon: d.Icon, FolderPath: d.FolderPath})
+			pos = len(out) - 1
+			index[sourceID] = pos
+		}
+		if len(out[pos].Blocks) < maxBacklinkBlocks {
+			out[pos].Blocks = append(out[pos].Blocks, BacklinkBlock{ID: blockID, Content: content})
+		} else {
+			out[pos].More++
+		}
+	}
+	sort.SliceStable(out, func(i, j int) bool { return strings.ToLower(out[i].Title) < strings.ToLower(out[j].Title) })
+	return out, nil
+}