feat(inventory-go): wire spell filters into /search/items; harden empty-input guards

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

go-services/inventory-go/search.go

@@ -302,6 +302,9 @@ func (s *Server) runSearch(ctx context.Context, q url.Values) (map[string]any, e
 		}
 	}
 
+	// --- spell filters: has_spell / spell_contains / legendary_cantrips ---
+	conds = append(conds, spellFilterConds(q, s.spells, ab)...)
+
 	where := ""
 	if len(conds) > 0 {
 		where = " WHERE " + strings.Join(conds, " AND ")