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/entities.go

@@ -152,6 +152,10 @@ func (s *Store) Create(ctx context.Context, e *Entity) error {
 		return err
 	}
 
+	if err := syncLinks(ctx, tx, s, e.ID, e.Body); err != nil {
+		return err
+	}
+
 	return tx.Commit()
 }
 
@@ -383,6 +387,12 @@ func (s *Store) Update(ctx context.Context, id string, u *EntityUpdate) error {
 		}
 	}
 
+	if u.Body != nil {
+		if err := syncLinks(ctx, tx, s, existing.ID, *u.Body); err != nil {
+			return err
+		}
+	}
+
 	return tx.Commit()
 }
 
@@ -495,6 +505,10 @@ func (s *Store) Absorb(ctx context.Context, targetID, sourceID string) error {
 		}
 	}
 
+	if err := syncLinks(ctx, tx, s, targetID, merged); err != nil {
+		return err
+	}
+
 	if source.CardType != nil {
 		if _, err := tx.ExecContext(ctx, `UPDATE entities SET card_type = NULL, card_data = NULL,
 			use_count = 0, last_used_at = NULL, modified_at = ? WHERE id = ?`,