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/tui/model.go

@@ -287,6 +287,12 @@ func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 		m.detail.mode = detailPreview
 		return m, tea.Batch(loadEntities(m.store, m.listParams()), m.setStatus("copied resolved"))
 
+	case backlinksLoadedMsg:
+		if m.detail.entity != nil {
+			m.detail.backlinks = msg.backlinks
+		}
+		return m, nil
+
 	case tagsLoadedMsg:
 		m.filter.setTags(msg.tags)
 		m.tagRail.setTags(msg.tags)
@@ -865,6 +871,7 @@ func (m model) updateBrowse(msg tea.KeyMsg) (tea.Model, tea.Cmd) {
 				} else {
 					m.state = stateDetail
 				}
+				return m, loadBacklinks(m.store, e.ID)
 			}
 		}
 		return m, nil
@@ -1212,6 +1219,7 @@ func (m model) selectedEntity() *db.Entity {
 func (m model) reloadDetail(id string) tea.Cmd {
 	return tea.Batch(
 		loadEntities(m.store, m.listParams()),
+		loadBacklinks(m.store, id),
 		func() tea.Msg {
 			e, err := m.store.Get(context.Background(), id)
 			if err != nil {