feat(db): add wiki-link extraction, resolution, and backlinks

[[wiki-links]] in entry bodies are extracted at save time, resolved
to entity IDs (title match first, body substring fallback), and
stored in entity_links junction table. Backlinks surface in TUI
detail view showing entries that link to the current entry.

Schema migration v5 adds entity_links with CASCADE/SET NULL
semantics. Links sync on Create, Update, and Absorb.

internal/db/links.go

@@ -0,0 +1,83 @@
+package db
+
+import (
+	"context"
+	"database/sql"
+	"strings"
+
+	"github.com/lerko/nib/internal/link"
+)
+
+type Backlink struct {
+	EntityID string
+	Title    *string
+	Body     string
+	LinkText string
+}
+
+func (s *Store) resolveLink(ctx context.Context, tx *sql.Tx, linkText string, excludeID string) *string {
+	lower := strings.ToLower(linkText)
+
+	var id string
+	err := tx.QueryRowContext(ctx, `
+		SELECT id FROM entities
+		WHERE LOWER(title) = ? AND id != ? AND deleted_at IS NULL
+		ORDER BY created_at DESC LIMIT 1`, lower, excludeID).Scan(&id)
+	if err == nil {
+		return &id
+	}
+
+	err = tx.QueryRowContext(ctx, `
+		SELECT id FROM entities
+		WHERE LOWER(body) LIKE ? AND id != ? AND deleted_at IS NULL
+		ORDER BY created_at DESC LIMIT 1`, "%"+lower+"%", excludeID).Scan(&id)
+	if err == nil {
+		return &id
+	}
+
+	return nil
+}
+
+func syncLinks(ctx context.Context, tx *sql.Tx, s *Store, entityID string, body string) error {
+	if _, err := tx.ExecContext(ctx, "DELETE FROM entity_links WHERE from_id = ?", entityID); err != nil {
+		return err
+	}
+
+	linkTexts := link.ExtractLinks(body)
+	for _, lt := range linkTexts {
+		toID := s.resolveLink(ctx, tx, lt, entityID)
+		if _, err := tx.ExecContext(ctx,
+			"INSERT OR IGNORE INTO entity_links (from_id, to_id, link_text) VALUES (?, ?, ?)",
+			entityID, toID, lt); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (s *Store) LoadBacklinks(ctx context.Context, entityID string) ([]Backlink, error) {
+	rows, err := s.db.QueryContext(ctx, `
+		SELECT e.id, e.title, e.body, el.link_text
+		FROM entity_links el
+		JOIN entities e ON e.id = el.from_id
+		WHERE el.to_id = ? AND e.deleted_at IS NULL
+		ORDER BY e.created_at DESC`, entityID)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	var backlinks []Backlink
+	for rows.Next() {
+		var bl Backlink
+		var title sql.NullString
+		if err := rows.Scan(&bl.EntityID, &title, &bl.Body, &bl.LinkText); err != nil {
+			return nil, err
+		}
+		if title.Valid {
+			bl.Title = &title.String
+		}
+		backlinks = append(backlinks, bl)
+	}
+	return backlinks, rows.Err()
+}