feat(phase-4): WS1 — Production Hardening (Redis rate limiting, DB pool, health endpoint, k6)

Rate limiting:
- Replace in-memory express-rate-limit with ioredis + rate-limiter-flexible (sliding window)
- Graceful fallback to RateLimiterMemory when Redis unreachable
- RATE_LIMIT_WINDOW_MS / RATE_LIMIT_MAX_REQUESTS env var config
- Retry-After header on 429 responses
- agentidp_rate_limit_hits_total Prometheus counter

Database pool:
- Explicit pg.Pool config via DB_POOL_MAX/MIN/IDLE_TIMEOUT_MS/CONNECTION_TIMEOUT_MS
- Defaults: max=20, min=2, idle=30s, conn timeout=5s
- agentidp_db_pool_active_connections + agentidp_db_pool_waiting_requests gauges

Health endpoint:
- GET /health/detailed — per-service status (database, Redis, Vault, OPA)
- healthy / degraded (>1000ms) / unreachable classification
- HTTP 200 (all healthy) / 207 (any degraded) / 503 (any unreachable)

Load tests:
- tests/load/ with k6 scenarios for agent registration (100 VUs), token issuance (1000 VUs), credential rotation (50 VUs)
- npm run load-test script

Tests: 586 passing, zero TypeScript errors

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -12,7 +12,8 @@
     "test:integration": "jest tests/integration",
     "db:migrate": "ts-node scripts/migrate.ts",
     "lint": "eslint src --ext .ts",
-    "format": "prettier --write src/**/*.ts"
+    "format": "prettier --write src/**/*.ts",
+    "load-test": "k6 run tests/load/agent-registration.js && k6 run tests/load/token-issuance.js && k6 run tests/load/credential-rotation.js"
   },
   "dependencies": {
     "@open-policy-agent/opa-wasm": "^1.10.0",
@@ -23,6 +24,7 @@
     "dotenv": "^16.4.5",
     "express": "^4.18.3",
     "helmet": "^7.1.0",
+    "ioredis": "^5.10.1",
     "joi": "^17.12.3",
     "jsonwebtoken": "^9.0.2",
     "kafkajs": "^2.2.4",
@@ -34,6 +36,7 @@
     "pino": "^8.19.0",
     "pino-http": "^9.0.0",
     "prom-client": "^15.1.3",
+    "rate-limiter-flexible": "^5.0.5",
     "redis": "^4.6.13",
     "ulid": "^3.0.2",
     "uuid": "^9.0.1",