fix: check Redis connectivity in /ready health probe

[Jayycob2211]

Overview

This PR closes a readiness gap where /ready reported healthy even when Redis was unreachable. A pod with a dead cache connection would keep receiving traffic from k8s, and any GET /vaults request hitting the cache path would throw 500 and fall back to the DB, causing a thundering herd. The /ready probe now pings Redis (with a 500ms timeout) and returns 503 when it fails, while treating Redis as optional when REDIS_URL is not configured.

Related Issue

Changes

🩺 Redis Readiness Check

• [MODIFY] backend/src/redisCache.ts

  • Expose a ping() helper that performs a Redis PING with a 500ms timeout so the health probe can verify cache connectivity.
  • Report whether Redis is configured (i.e. REDIS_URL set) so callers can distinguish "optional and absent" from "configured but down".

• [MODIFY] backend/src/healthProbe.ts

  • Add a redis entry to the /ready checks.
  • When Redis is configured, run redis.ping(); on failure return 503 with checks: { redis: { status: 'down' } }.
  • When REDIS_URL is not set, report checks: { redis: { status: 'up', optional: true } } instead of down.

• [MODIFY] backend/src/__tests__/healthReady.test.ts

  • Update /ready contract tests to assert the new redis check is present.
  • Add a test that mocks redis.ping to throw and asserts /ready responds 503.
  • Add a test covering the optional-Redis case (REDIS_URL unset → up with optional: true).

• [MODIFY] backend/schema-snapshots/get-_ready.json

  • Refresh the /ready response snapshot to include the redis check.

Verification Results

npm test -- backend/src/__tests__/healthReady.test.ts
✅ healthReady tests passed (including redis-down → 503 and optional-redis → up)

Acceptance Criteria	Status
Add redis.ping() check with 500ms timeout to /ready; return 503 with checks: {redis: {status: 'down'}} on failure	✅ Implemented in healthProbe.ts / redisCache.ts
Optional redis (when REDIS_URL not set) reports up with optional: true	✅ Covered by updated contract test
Update health/ready contract tests and snapshots to include redis check	✅ healthReady.test.ts + get-_ready.json updated
Test: mock redis.ping to throw and assert /ready is 503	✅ Added to healthReady.test.ts

Closes #1443

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