feat: replace readiness probe with httpGet when instance manager enabled

[usiegj00]

Summary

Replaces the process-spawning readiness probe with an HTTP probe to /readyz:8080 when instance manager is enabled.

Fixes #8

Changes

Condition	Readiness Probe
instanceManagerImage set	httpGet /readyz:8080
instanceManagerImage not set	exec ready.sh (legacy)

Operation Comparison: ready.sh vs /readyz

Condition	ready.sh	/readyz
Master role	✅ Always ready	✅ Always ready (unless loading)
loading:1	❌ Not checked	✅ NOT ready
master_sync_in_progress:1	✅ NOT ready	✅ NOT ready
master_host:127.0.0.1	✅ NOT ready	✅ NOT ready
master_link_status:down	❌ Not checked	✅ NOT ready

The /readyz endpoint covers all ready.sh checks plus additional safety checks.

Before (exec probe)

readinessProbe:
  exec:
    command: ["/bin/sh", "/redis-readiness/ready.sh"]

Problems:

• Spawns new process every 10 seconds
• Script startup adds latency
• Fails under memory pressure

After (httpGet probe)

readinessProbe:
  httpGet:
    path: /readyz
    port: 8080

Benefits:

• No process spawning
• ~1-5ms latency
• Works under memory pressure
• Persistent Redis connection with cached status

Backwards Compatibility

• Only applies when instanceManagerImage is set
• Legacy exec probe used when instance manager not enabled
• No breaking changes for existing deployments

Test Plan

• Unit tests for HTTP probe when instance manager enabled
• Unit tests for exec probe when instance manager disabled
• Unit test for no-master-configured check
• E2E tests verify httpGet readiness probe configuration