perf(load-tests): run K6 baseline and fix search.js URLSearchParams bug

- Fix search.js: replace URLSearchParams (unsupported in K6) with string interpolation
- Add baseline performance report with latency benchmarks across all 4 suites
- Add load-tests/results/*.json to .gitignore (large raw output files)

Note: pre-existing test failure in create-listing.handler.spec.ts (eventBus.publish mock) — unrelated to this change.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

.gitignore

@@ -29,6 +29,7 @@ coverage/
 playwright-report/
 test-results/
 blob-report/
+load-tests/results/*.json
 
 # misc
 *.log