feat(load-tests): add K6 load testing suite for critical API paths

K6 scripts for 4 critical paths:
- Auth (100 VU): login, register, profile
- Listings (500 VU): search with filters, detail view
- Search (200 VU): full-text + geo search
- Payments (50 VU): create payment, list transactions

SLA thresholds: p50<200ms, p95<500ms, p99<1s, error<1%.
CI: manual workflow_dispatch with suite selector.

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

load-tests/README.md

@@ -0,0 +1,72 @@
+# K6 Load Testing — Goodgo Platform
+
+## Overview
+
+Performance load tests for critical API paths using [K6](https://k6.io/).
+
+## Test Suites
+
+| Script | Target | Peak VUs | Duration |
+|--------|--------|----------|----------|
+| `auth.js` | Login/Register | 100 | 2min |
+| `listings.js` | Search + Detail | 500 | 3min |
+| `search.js` | Text + Geo search | 200 | 3min |
+| `payments.js` | Create + List | 50 | 2min |
+
+## SLA Thresholds
+
+| Metric | Threshold |
+|--------|-----------|
+| p50 latency | < 200ms |
+| p95 latency | < 500ms |
+| p99 latency | < 1000ms |
+| Error rate | < 1% |
+
+## Prerequisites
+
+```bash
+# Install K6
+brew install k6        # macOS
+# or: https://grafana.com/docs/k6/latest/set-up/install-k6/
+
+# Start the API
+pnpm --filter @goodgo/api run dev
+```
+
+## Running Tests
+
+```bash
+# Run individual suite
+k6 run load-tests/scripts/auth.js
+k6 run load-tests/scripts/listings.js
+k6 run load-tests/scripts/search.js
+k6 run load-tests/scripts/payments.js
+
+# Run against a custom API URL
+k6 run -e API_BASE_URL=https://staging.goodgo.vn load-tests/scripts/auth.js
+
+# Run all suites sequentially
+for f in load-tests/scripts/*.js; do k6 run "$f"; done
+
+# Export results as JSON
+k6 run --out json=results/auth.json load-tests/scripts/auth.js
+```
+
+## CI Integration
+
+The `load-test.yml` workflow runs as an optional manual stage in GitHub Actions.
+Trigger via `workflow_dispatch` with a suite selector.
+
+## Directory Structure
+
+```
+load-tests/
+├── lib/
+│   └── config.js          # Shared config, helpers, SLA thresholds
+├── scripts/
+│   ├── auth.js            # Auth flow load tests
+│   ├── listings.js        # Listings search + detail
+│   ├── search.js          # Full-text + geo search
+│   └── payments.js        # Payment creation + listing
+└── README.md
+```