feat(load-tests): add K6 coverage for search, admin, and MCP endpoints

Add three new K6 load test scripts to cover previously untested API surfaces:

- search-advanced.js: Combined geo + text + filter queries, paginated deep
  search, and sort variations against /search and /search/geo (300 peak VUs)
- admin.js: Moderation queue CRUD, bulk moderation, dashboard stats, audit
  logs, and user management endpoints (50 peak VUs)
- mcp.js: MCP server discovery, SSE connection, property-search tool calls,
  valuation/batch-valuation, and feature extraction (120 peak VUs)

Also updates README with new suite documentation, per-suite custom thresholds,
and adds the new suites to the CI workflow_dispatch selector.

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

.github/workflows/load-test.yml

@@ -13,6 +13,9 @@ on:
           - auth
           - listings
           - search
+          - search-advanced
+          - admin
+          - mcp
           - payments
 
 concurrency: