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/commands.go

@@ -55,6 +55,10 @@ type stepsPersistedMsg struct{}
 
 type templateCopiedMsg struct{}
 
+type backlinksLoadedMsg struct {
+	backlinks []db.Backlink
+}
+
 type tagsLoadedMsg struct {
 	tags []db.TagCount
 }
@@ -194,6 +198,16 @@ func loadTags(store *db.Store) tea.Cmd {
 	}
 }
 
+func loadBacklinks(store *db.Store, entityID string) tea.Cmd {
+	return func() tea.Msg {
+		backlinks, err := store.LoadBacklinks(context.Background(), entityID)
+		if err != nil {
+			return errMsg{err}
+		}
+		return backlinksLoadedMsg{backlinks}
+	}
+}
+
 func loadRailTags(store *db.Store) tea.Cmd {
 	return func() tea.Msg {
 		tags, err := store.ListTags(context.Background(), false)