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>

.github/workflows/load-test.yml

@@ -0,0 +1,102 @@
+name: Load Tests (K6)
+
+on:
+  workflow_dispatch:
+    inputs:
+      suite:
+        description: 'Test suite to run'
+        required: true
+        default: 'all'
+        type: choice
+        options:
+          - all
+          - auth
+          - listings
+          - search
+          - payments
+
+concurrency:
+  group: load-test-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  load-test:
+    name: K6 Load Test — ${{ inputs.suite }}
+    runs-on: ubuntu-latest
+    timeout-minutes: 20
+
+    services:
+      postgres:
+        image: postgis/postgis:16-3.4
+        env:
+          POSTGRES_DB: goodgo_test
+          POSTGRES_USER: goodgo
+          POSTGRES_PASSWORD: goodgo_test_secret
+        ports:
+          - 5432:5432
+        options: >-
+          --health-cmd "pg_isready -U goodgo -d goodgo_test"
+          --health-interval 10s
+          --health-timeout 5s
+          --health-retries 5
+
+    env:
+      DATABASE_URL: postgresql://goodgo:goodgo_test_secret@localhost:5432/goodgo_test
+      NODE_ENV: test
+      JWT_SECRET: load-test-jwt-secret-key
+      JWT_REFRESH_SECRET: load-test-refresh-secret-key
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Install K6
+        run: |
+          sudo gpg -k
+          sudo gpg --no-default-keyring --keyring /usr/share/keyrings/k6-archive-keyring.gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys C5AD17C747E3415A3642D57D77C6C491D6AC1D68
+          echo "deb [signed-by=/usr/share/keyrings/k6-archive-keyring.gpg] https://dl.k6.io/deb stable main" | sudo tee /etc/apt/sources.list.d/k6.list
+          sudo apt-get update
+          sudo apt-get install k6
+
+      - name: Install pnpm
+        uses: pnpm/action-setup@v4
+
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: pnpm
+
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+
+      - name: Run database migrations
+        run: pnpm db:migrate:deploy
+
+      - name: Build and start API
+        run: |
+          pnpm --filter @goodgo/api run build
+          pnpm --filter @goodgo/api run start &
+          sleep 10
+
+      - name: Run load tests
+        run: |
+          mkdir -p results
+          if [ "${{ inputs.suite }}" = "all" ]; then
+            for f in load-tests/scripts/*.js; do
+              name=$(basename "$f" .js)
+              echo "=== Running $name ==="
+              k6 run --out json="results/${name}.json" "$f" || true
+            done
+          else
+            k6 run --out json="results/${{ inputs.suite }}.json" \
+              "load-tests/scripts/${{ inputs.suite }}.js"
+          fi
+
+      - name: Upload results
+        if: ${{ !cancelled() }}
+        uses: actions/upload-artifact@v4
+        with:
+          name: k6-results-${{ inputs.suite }}
+          path: results/
+          retention-days: 30