feat: replace liveness probe with httpGet when instance manager enabled

[usiegj00]

Summary

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

Fixes #7

Changes

Condition	Liveness Probe
instanceManagerImage set	httpGet /healthz:8080
instanceManagerImage not set	exec redis-cli ping (legacy)

Before (exec probe)

livenessProbe:
  exec:
    command: ["sh", "-c", "redis-cli ... ping | grep PONG"]

Problems:

• Spawns new process every 15 seconds
• redis-cli startup adds ~50ms latency
• Fails under memory pressure

After (httpGet probe)

livenessProbe:
  httpGet:
    path: /healthz
    port: 8080

Benefits:

• No process spawning
• ~1-5ms latency
• Works under memory pressure

Backwards Compatibility

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

Test Coverage

• Unit tests for HTTP probe when instance manager enabled
• Unit tests for exec probe when instance manager disabled
• E2E tests verify httpGet probe configuration
• E2E tests verify health port is exposed

Test Plan

• Unit tests pass
• E2E tests pass
• httpGet probe configured when instance manager enabled
• exec probe used when instance manager disabled
• Health port 8080 exposed in container spec